No whitelist application as yet. It may be possible to construct one.
(Edit: See this thread: http://talk.maemo.org/showthread.php?t=34392 , for example)

Within the constraints of being a phone, the N900 is pretty much a full linux system, with some caveats. That's not to say the interface is like a desktop interface, it's not, as you can see from the many youtube vids showing the N900 UI, but it's a fairly unrestricted linux underneath that.