Randy announced a while back that a new signature policy would go into effect today that requires 90 days and 90 posts to have a signature. You'll find the policy here:

http://www.flyertalk.com/help/rules.php#q79

I believe that your signature is still in your profile; the software suppresses it from being visible. I am guessing here, but I expect that when you reach both those milestones, your signature will become visible once again.