based on my own experiences, many macbook pros are finicky when it comes to wireless routers. they'll work well with some and not others.
while it's tempting to say "it must be a junk router," the mbp will often be the *only* device that can't connect to it. moreover, several times it's happened to me that the problem will suddenly go away (after some weeks/months of frustration) without having modified the router or mbp networking settings.
i kinda suspect osx updates might be the culprit; the problem seemingly got better on my home network around 10.6.4, but then started up at work just after 10.6.6.