Not sure it's relevant as your profile shows you're running Snow Leopard, but you might want to have a look at this link - and the links within it. Seems to have helped some people using Lion and Mountain Lion. Maybe it'll offer some ideas you can try.
Another possibility could be that the Keychain entry is corrupt. You could try deleting it from the and letting it create a new entry.
Utilities>Keychain Access>Passwords. Find the Network entry and delete. Try to rejoin the Network and it should create a new Keychain item.
Change your router channel. Sometimes this is all you will have to do.
Power cycling the router. Read the router's user manual or contact their tech support for instructions.
System Preferences/Internet & Network/Network
Unlock the padlock
Click the Assist Me button
In the popup window click the Diagnostic button.
System Preferences/Network- Unlock padlock. Highlight Airport. Network Name-select your name. Click on the Advanced button. Airport/Preferred Networks-delete all that is not your network.
Place a check mark next to "Remember networks this computer has joined." Click the OK button and lock the padlock. Restart your computer.
http://support.apple.com/kb/TS1920 Mac OS: How to release and renew a DHCP lease
No internet connection (wireless)
Check to see if an extra entry is present in the DNS Tab for your wireless connection (System Preferences/Network/Airport/Advanced/DNS).
Delete all extra entries that you find.
Place a check mark next to "Remember networks this computer has joined."
Other resources to check into: