I know for a fact that a person used to be able to install bitcoind on their N900
He used linux port on his N900, not a native firmware, so that doesn't counts.
Don't know how it doesn't count as native but here's the thread: https://bitcointalk.org/index.php?topic=2125.0
Downloading the chain takes a long time and the phone heats up while doing it. Afterwards though, it worked like a charm. I'm sure with the new improvements on chain downloading, it should run better, assuming you can build it. Either way, it doesn't make much sense to run bitcoind on an N900, even though it's pretty cool and I keep my installation just in case.
As a GUI, something like Spesmilo should theoretically work out of the box (never tried myself). Then again, I would prefer connecting to my bitcoind at home rather than running it on the phone.