i did not yet find an easily read and understood chart of which parts of the n900's hardware are _not_ open.

coming from the freerunner i know we had some issues mostly with the gsm chip (it took some bothering until the producer provided a fixed firmware) and the graphics chip (specs available only under nda, although the chip being discontinued), though the gsm is encapsulated and communicates via at commands and the graphics chip received some heavy reverse engineering, pushing it somewhat over the limits intended by the manufacturer.

iirc there are some crucial parts which have closed drivers only, meaning once nokia loses interest in the n900 (which seems to have happened already, given the hassle with meego), we are pretty much screwed.
let's hope either nokia opens the specs and makes free drivers possible or there are enough hw guys to reverse engineer those parts.