Bluetooth sounds good, but what chips/components would you use to connect e.g. the HMC5843 compass chip (which has an I2C interface) with a Bluetooth module? SparkFun has a BlueSMiRF Gold Bluetooth module that is pretty small, but it seems to take RS232 as input, not the I2C that the HMC5843 has. How would you interface the two? I found an I2C-to-RS232 module, which could then connect the I2C device to the BlueSMiRF, but the adapter module is as huge as a brick. That company says they are considering making an I2C-to-RS232 chip, but they haven't yet. I also found a supposed I2C-to-Bluetooth module, but I couldn't find it for sale anywhere.

There also seems to be an RS232 compass module which could perhaps directly be hooked up to the BlueSMiRF. Minimum order quantity is 50, though.

Then there's also the issue of power - hooking up some batteries to this will increase the size even further...

I've got another really, really, REALLY low-tech idea about how to do this, but I'll only consider that if all else fails. Idea: use the low-res, user-facing camera to do image processing on an analog compass and determine its orientation This then becomes more a problem of mechanical rather than electrical engineering...

EDIT: Another potential compass module with RS232 interface that might be connectable to BlueSMiRF: TD-CMP03. Kind of large at 40x41mm. It has tilt compensation; I wonder if it can be held vertically upright against the N900 body, or if it has to be held horizontally. No word on its power consumption. Again, comment from electronics gurus would be welcomed... is it really as simple as connecting the RX/TX lines from the compass to a BlueSMiRF, and connecting the power?