Is it possible to measure/monitor/record the signal strength of the 3G radio as well as the 3.5G (HSPA) radio? They operate on different frequencies, so it seems plausible.

I would like an application that logs GPS coordinates and both signal strengths. I could then document how much my coverage fluctuates at work (a LOT) and make my own map of dead zones, etc.

sorry but the phone doesn't report the signal strength for each band individually. I don't think you can disable 3.5G without disabling 3G either, and the source is closed so you can't see how it works

Err.... its the same radio and the same frequency. Whether or not you receive 3.5g or 3g depends on the tower you are connected to.

And also your service provider and their throttling settings.

The same is true for 2G and 2.5G.

I don't really agree with the previous statements. They are either incorrect or unclear.
3G requires an uplink and downlink, usually on seperate bands, but some providers just use a single band.
3.5G = High-Speed Downlink Packet Access, this means the n900 had a 3G connection and has temporarily established additional data channels.
n900 does not support MIMO or dual-cell HSDPA, but future phones supporting dual cell will be using two seperate frequency bands to double the bandwidth of the downlink. MIMO just connects with two antennas to the same band.
Anyways, if the uplink on the n900 is on a seperate band, and interference causes it to be lost, the phone will probably drop to 2.5G, if available. Actually it's hopefully EDGE (2.75G) in most areas, n900 doesn't display 2.75G though, and i'm not sure if it has an edge symbol on the display, but anyways n900 definately supports GSM frequencies so it should work on some providers.

You are correct that most carriers use a different frequency for uplink and downlink; however, 3G and 3.5G operate on the same frequency (for incoming and outgoing data, respectively) since it is technically based on the same technology.

So to go back to the original question, you can measure 3G signal using app like Netmon but you cannot measure 3G and 3.5G separately since the tower will only connect you to one standard at a time.

3G and 3.5G *can* operate on different frequencies, but don't necessarily do so. It just requires that the operator has access to multiple channels and decides the configure the network to make it so.

3G is not really a specific standard, it's more like a bunch of technologies that include UMTS, W-CDMA, HSPA+. So both the uplink and downlink are 3G, but only the downlink is 3.5G. The uplink doesn't have a specific "G" name because it's mostly the marketing people that come up with this **** and nobody cares how fast their phone can upload data... (2mbit for ours if you're wondering).
I don't know how the hardware radios work, but I thought maybe if it's connected to different bands, it could report the signal strength for each one individually. If they had programmed an interface for that, which they didn't do.