I found this on another site..it might be worth a try as I'm running out of ideas.....
"An iPad bought a few days ago connected fine to a Linksys (Cisco) router, then yesterday refused to negotiate a valid IP address via DHCP handshake. No amount of resetting, restarting, or rebooting--even restoring--either the router, cable modem, or iPad resulted in success. (This included forgetting networks and renewing leases.) All other devices in the network connected and re-connected flawlessly; conversely, the iPad would connect to other wi-fi networks.
The router was put through several changes, including choosing only 11g devices, and changing transmission channels & SSID names. Finally, what worked a mere seconds ago, was changing the security protocol from WPA2/AES+TKIP to WPA/TKIP (only). That kickstarted the iPad into a valid IP address. In a fit of impulsivity, I changed the security BACK to WPA2/AES+TKIP and then to AES (only), and the iPad continues to connect appropriately, even after awakening from sleep mode.
I'm not sure what happened, but my guess is that the IP lease expired, and the iPad refused to negotiate a new address with the router, until the security change forced something loose. In any case, this seems to be a prevalent issue, judging from the number and breadth of threads clamoring about this issue in many forms, so this just a note of what worked for me."