If you've been messing around with your security settings, try deleting all references to your wireless network on your mac. Delete the network from the "preferred networks" list in Network Sys Prefs. Go into Keychain Access and delete all references to it in there. Then re-connect. I've seen OSX get borked a numbed of time when you change the security type or settings on a network that it already has stored as a favourite.

e.g.

Connect to a WEP network, and add it to your keychain.
Change the WEP to a WPA network on your wireless router.
OSX will now have a bit of a fit whenever you connect, until you delete all references to it and connect again.

If that doesn't work, try running it with no encryption.

This thread seems to be about as popular as a root canal, but if it helps out just one other person...

Its appears that the 10.4.10 update has screwed a few peoples wireless internet connections, particularly on MBs. The problems can range from intermittent wireless connection through to frequency KP action. From what I can gather its something to do with what the airport card does when there's no traffic... some sort of idling problem. If I have a big download going constantly then i can surf away for hours with no problems.

Anyway, I hope those bright sparks at apple dish out a patch soon.