Now that the N900 supposedly has IR capability, that's one more connection type to consider. IrDA transmitter/receiver is typically directly connected to a UART, which is really easy to access in Linux.

You would need a clip to keep the IR LED aligned with the receiver on the N900, but software-wise it seems the simplest option. Hardware would consist of a simple 8-bit 8-pin microcontroller ($2 or less a piece) with an internal AD-converter and some support components. Atmel Tiny AVR series is a good fit, and the compiler suite (gcc) is free and runs on Linux. AVR gcc could even be made to run on the tablet if desired...

(Disclaimer: I'm an EE and do this stuff for living.)