I have been wondering for 2 days since I received my N900 why charging seems hit and miss from non-PC based USB ports, for example car cigarette lighter USB adapters and USB Battery packs. Here's the answer and solution.

The Micro USB spec says that power draw should be under 100mA until the phone and connected power source device (ie laptop, computer, etc) can negotiate a higher current. This negotiation happens over the data pins (the center ones in the big USB plug). If the power source is a "dumb" device which is not capable of negotiation (for example the dedicated wall socket charger), the spec says that the data pins (D+, D-) should be shorted - indicating to the phone that it can draw as much power as it needs (up to 1A?)

My problem is that several USB Power sources that I have (a few car cigarette lighter to USB adaptors, and an APC Portable battery), have the center pins open. The phone therefore tries to negotiate with the dumb power source and obviously fails, and hence proceeds to ignore it.

There are a couple of solutions. For cigarette lighter to USB adaptors, you can probably just pull them apart and solder the 2 center pins of the USB socket together on the PCB. Another solution would be to cut your USB to micro USB cable in the middle, short the D+ and D- (usually white and blue wires from memory - check with a multimeter first!) and neatly join/heatshrink the cut. This would render the cable useless for anything except charging from the dumb power source.

In my case I neither wanted to sacrifice a USB cable, nor did I want to short the pins on the USB socket of my portable battery, as I wanted to retain its "passthrough mode", which transparently passes data through from a PC. Shorting the D+ and D- pins would destroy the passthrough mode.

Instead I salvaged a male to female USB part from an old USB hard disk enclosure (the cable that used to provide extra power to the USB hard disk). I cut the power cable off it, and heatshrinked the wires so they won't short. I then carefully soldered the center two conductors together in the USB socket.

Voila - a "Force Dedicated Charging" USB adaptor, for dumb USB power sources.

Attached are photos. (incidentally, taken with the n900 camera in macro mode - stunning. I had to downscale them for the forum, but the originals are here if anyone is interested)

I wouldn't recommend you plug this adaptor into a PC - although it probably would do no harm as the ports are meant to be current limited, but shorting the data pins together will obviously terminate any communication on the USB bus.

UPDATE:
A lot of people have pointed out that the FM transmitter output power is reduced when plugging any sort of charger in (rendering the FM transmitter close to useless). Nokia have implemented this power reduction due to the fact that the charge cable will act as a transmit antenna, which brings the possibility that the transmit power will be well over legal limits.
If you would like to over-ride this power limitation at your own risk, with the possibility that you'll be transmitting at radio power levels over the legal limit, then grab the modified fmtxd binary from here:
http://jacekowski.org/Maemo/FMTXD
or here (zipped, post 8):
http://talk.maemo.org/showthread.php?t=60567

Make sure FM transmitter is off, then copy the new fmtxd to your n900 (root of the mass storage), then run the following in terminal:
You should now be able to charge as well as simultaneously transmit (possibly illegally) on FM.

UPDATE 2:
Added photo of my current solution (last photo in below series). These weren't hard to find on ebay, (usb male to female adaptor) and the same solder bridge trick was applied. Much neater than original hack.

Is it possible to buy a cable that has the pins shorted? How would they be idenitified?

Definitely won't be possible to buy a cable with the pins shorted.

Let me be clear that this cable/adapter is USELESS for transferring data, or for charging from a PC USB port.

The sole purpose of this hack is to force the phone to think that it's plugged into a dedicated charger (which I call a dumb power source).

You only need to think about doing something like this if you have a USB socket which the phone will not charge from (and you want it to).

For everybody else, I recommend you buy dedicated wall chargers and dedicated car chargers - these will already have the D+ and D- pins internally shorted, and never need to transfer data.

Cool. I just opened up my USB charger lighter adapter, and bridged the two inner data terminals as you suggest, and now my N900 will charge from it. Unfortunately, the FM transmitter becomes very weak when connected.

This aspect of the N900 is very lame indeed - it works fine when not charging, but why on earth would I want to flatten the device's battery when I'm listening to it through the car stereo?

I haven't had a chance to experiment with the FM transmitter yet, but I would assume this is a hardware limitation, possibly to do with grounding or something that occurs which reduces the efficiency of the radio transmitter. It's early days: it'll be interesting to see what hacks people come up with to solve this problem.
UPDATE: told ya! see solution in opening post.

When I tried using the FM transmitter while plugged into a laptop, the n900 flat out told me that the FM Transmitter couldn't be active while USB is connected.

The FM transmitter's most obvious use case would be as an in-car music system, so it's indeed very strange that it won't work right when also connected to a car charger.

Sorry for going off topic here.

I've never seen that happen, and I've experimented a fair bit. I can always hear the FM TX on my handheld FM receiver when it is activated. I get about 3 metres range with no USB cable attached and a good fully-quieting signal (no hiss), depending on device and antenna orientation.

When I plug the wall charger, or my newly functional car charger in to the N900, the signal level drops drastically, such that when the N900 and FM radio are right next to each other, the radio still hisses and pops, although I can hear the transmitted signal.

Yes, these cables exist but they are usually built as 2-into-1, so you can pool the available amperage from two USB ports to power a single device.

You just saved me 30 bucks!

I have one of those 12V Cigarette adapters that has 2 USB ports on it. I could never get to work with my N900.

So, I popped it open, heated up my handy dandy soldering iron, jumped the two center pins on each USB port on the circuit board, re-assembled it, and Vioala! The N900 charges!

Thanks again!