This is a good point. Some research on whether this device is obtainable is clearly in order.

All it takes is an I2C interface and you're in business. You can find the AK8973 datasheet on the web, e.g.,
http://www.datasheetarchive.com/pdf-.../DSA-37871.pdf
It's pretty clear from this that the interface portion is trivial.

I assume you went with the Honeywell 3-axis magnetometer? I'm fairly confident the N900's tilt sensors can be used to do the tilt compensation, so the much more expensive tilt compensated one shouldn't be necessary.

In my mind, the question of AK8973 vs. HMC5843 isn't a useful one because they are substantially the same. Ideally, we'd be able to tap into an existing i2c bus and be done with it---we just need the IC and some bypass caps. Going through USB (assuming OTG support becomes available) will probably require a microcontroller and will be bulkier and uglier.

By the way, as nice as the SDIO idea sounds, it's probably no better than doing a USB interface, since SDIO<->I2C will still take a microcontroller, and the issue of mechanically supporting the compass module protruding from an SD slot is not one I want to tackle. USB seems better suited to this. (I say protruding with fair certainty having played around with the HMC5843 enough to ascertain that it is too thick to go inside said slot. The AK8973 is somewhat thinner, but is still too thick when you consider that you need to mount it on a PCB.)