I installed BOINC on my desktop PC, it has an Intel Core 2 Duo E8400 running at 3.0 GHz (100% load) all the time for the Rosetta@Home and World Community Grid projects: it can complete a not-so-large task every 3-4 hours, but it needs some extra ventilation against the heat produced.

Now imagine all of that on the N900 :P It would complete a single task in a day (maybe), running at top speed all the time (so you could not use the phone in the meantime) with its 600 MHz CPU (overclocking is impossible, the heat produced would kill the phone in a couple hours).

Results: its contribute to the project would be nearly 0, and it would reduce its life significantly.