Having the same problem.
I'm using a linksys wrt54gc with wpa2 personal (aes) encryption. My connection is not hidden (broadcasting ssid). My fiance has an identical phone which has a similar problem (haven't done enough testing yet with it myself). It'll sometimes be able to connect, then not be able to at other times.
The phone will see it, connect to it and get as far as obtaining an ip address before returning 'unsuccessful'.
I've tried both assigning the phone a static ip and letting dhcp handle it, neither works.
Anyone have some suggestions?