First off, make sure you are entering the password right.
Second, you could go into the "Bios" or the Settings of the router, and change the password.
To get there, put in a web browser this address 192.168.2.1 , or 192.168.1.1 . If it asks for a username and password, try entering Admin as the username and Admin as the password (that is if you havn't changed, which I suggest you do)
If none of those work, you could wait for iOS 4.2 and see if that solves your problem.