I remember there had been talk to release sourcecode of Doom3-engine?
Would prBoom³ be possible?
Well, theoretically, yes. BUT, even if it was somehow magically compiled to work on the N900, it wouldn't look like anything you would want to play. If you say Doom3 running on old Voodoo II cards you know what i mean...
Doom 3 engine source has not even been released yet, though unless I've missed it. Last time I heard about was when Carmack was saying it's going to be delayed longer than what was the case with Q3.