The N900 is the way to go just for its flexibility:
- no complicated jail-breaking needed or fighting with upstream to keep control of your phone's internals
- can install a full Openssh stack or the lighter Dropbear if you don't need all features
- several VNC clients available
- full non-crippled browser
- custom kernel available enabling various networking and filesystem modules
- great contact manager with great VOIP connectivity (gtalk/SIP/Skype all even though 3G)
- recently available custom wireless driver that allows full security testing (packet injection, full moitor mode, etc.)
- and many more ...
While the N900 might be a bit too much of a power tool for someone looking for a simple phone that works well, it is ideal for a technical person with Linux knowledge that can really take advantage of the wide-open underlying Linux base.
My only gripe with this phone is the battery life. You have to charge it every single day. Not a huge deal since it can charge from any USB port but still annoying on extended trips.