I just click join another network and then enter my hidden SSID and type or paste in my WPA* security and it works again. I know it is annoying but it beats having to reboot the router or do other bizarre things.

History:
I also thought it was my router until I tested with my old Powerbook (Tiger) and my Ubuntu Thinkpad and both were still connecting fine.

I hope that helps others until the gremlin is found and shipped backed to Redmond.