Hmmm.....
I'd suggest resetting the router, and rebuilding the security profile from scratch.
Set it up for WPA (if all the devices will support it) and then ensure that all the wireless clients have the right settings.
It certainly sounds like a misconfiguration somewhere either on a client or on the router, I often find that setting up again from scratch is quicker than trying to work out what's gone wrong where.
I saw an interesting case a few months back, the firmware on the router had become corrupted and wasn't accepting any new clients. We had to flash the router's firmware to get the thing up and running properly again. Bit extreme though
