Happy to hint, and thanks for the diagram. Will be handy when I go to add it to my N900.

I was looking at that on the powermat site and debating it. I figured it was a little smaller than the iPhone pad, but wasn't sure it had other issues. Thicker sounds like a pain. :P

I've also been looking at various Qi based chargers, which should work the same way. They appear to have a slightly better range, and don't require the mag-lock powermat uses. (Plus, they're an industry standard that several folks are making pad/charger pairs for.) For right now I'm holding off a bit, since I'm over-project-ed as it is. But I do look forward to doing this soon.

Alright,

the new version works much better than the first one.
With the new receiver the problem of the charging being interrupted after some time is gone now. So far I successfully fully charged my N900 two times.

Though, there is a slight new "issue" the charging process is slower than via a USB connection to a PC right now. Afaik the problem is caused by the forward voltage drop of the diode.

Due to this the voltage on the battery side is smaller than 5V. Actually, the new Powermat receiver (Nintendo DS Lite) I am using supplies about 5.15V. Though, due to the diode in the circuit at the N900 USB side the voltage is lower. I already use Schottky diodes as these have the smallest forward voltage drop.

There are some solutions using a (MOS)FET instead of a diode which should result in a significantly lower voltage drop. Though, I am not that much into electrical engineering topics and didn't try this yet.

I'll keep you updated...

Alright,

I tried a solution with a different type of Schottky diode, one using a MOSFET as diode replacement and one connecting the +5V source directly. Neither of these worked. In all attempts the N900 started to charge, though, after some (10 or so) seconds the charging stopped. In some cases I could hear some chirping noise from the device before the charging stopped.
Apparently the BY 133 was a very fortunate choice for this setup.

I also made some pictures of the new receiver and the current setup I am using.
I'll upload the pictures as soon as I manage to get em off the camera.

As promised the pictures of the current setup using the Powermat Nintendo DS Lite receiver.

Edit: The Powermat Nintendo DS Lite receiver has a black, adhesive, circular protection foil on the back.
I reuse this foil in my setup as protection as well.
I just mention it here as it is not shown on the pictures.

Looks great. How much thickness does it add to the back? That's a major thing for me. (Could we get a photo from the side-on to get an idea?)

Also: I note this back has a lot more room, and isn't carved up. Where did you get your spare back? Most of the places I'm finding that have spare back panels are quite pricey.

Excuse the late reply... I am very busy at the moment.
With respect to the thickness: the first attempt on this added about 2-3 mm extra. Though, as it got loose I had to take a second try which now probably adds about 4-5 mm (?). Note that this only adds partially to the advertised overall thickness of the N900 as this includes afaik the bump of the camera slider and stand.
Still, this makes the device look even more clunky. But if you happen to like a diy-ish "MacGyver look" you probably wouldn't mind.

Also note that I experience problems with respect to the USB data connection. I didn't notice this earlier as I do not use this pretty often. I don't know if the problems are related to the optocoupler solution or if any USB related parts in the device got damaged. Unfortunately I am currently lacking the time to investigate this further. Charging via USB still works btw. IIRC the USB data connection did work with the first "dip-switch" solution.

Last but not least, if you like messing with your precious device in a similar way, be sure to check out the pretty informative post by joerg_rw at the bottom of the page: http://talk.maemo.org/showthread.php?t=71414&page=2

Edit:
One last note: I do not charge the device unattended, so I can quickly respond in case something goes wrong. Luckily so far I did not encounter such a circumstance.

I know this is an old thread but has anyone tried to use the inductive charger back cover from the Palm Pre with the Touchstone ? If you look at the cover the inductive device added to it is quite thin and looks like it would fit right under the cover of the N900. Also, upon closer inspection it appears it simply makes contact at the Palm Pre's existing + and - battery contacts for charging it's battery. Just a thought for whatever its worth.