I ran into this problem the other day and I thought it might help someone with similar issues.
I have a Linksys router for my 3 laptop house. Everything worked great with the Windows machines. Over the holidays my inlaws brought their ipad over but we couldn't get it to connect to my wifi using the WPA2 pass phrase. Every time I entered it it was rejected. Finally, after much mucking about, I logged in to my router and reduced the pass phrase from the insanely long one (32+ or whatever the max is) to something more manageable in the 9-15 character range and it worked. Can't say for sure if it was the long pass phrase or changing it on the router caused some other thing to reset but it worked. Of course, I then had to change it on all my other computers but c'est la vie!