Found an interesting link: Interfacing I2C sensors with the Nokia n800! The project page says "Attach any I2C client chip (thermo sensors, AD converter, displays, relais driver, ...) to your PC via USB". Compass (b) above that I linked to has I2C.

It says that you need to enable USB host on the n800 - which I think is not yet supported on the n900. Still, this looks like the most promising lead yet.