I, too, ran into this problem after upgrading to Snow Leopard. I had other email issues with Mail after the upgrade (wouldn't send/received, would close immediately after I sent a reply). Apple helped me fix the mail issues, but later found I couldn't get the "signatures: NONE" drop-down to activate (my signatures were never lost, I just couldn't get them to work). I followed Daniel's and 2point5's advice and it worked perfectly. Thanks!
Message was edited by: eobrien1528