We were down for three hours last night, about 9PM to midnight last night California time, because we ran out of our free CPU quota for the day. Google App Engine is still in beta, and until it's released we can't purchase more CPU time.
However, I submitted a request yesterday when I noticed we were probably going to go over, and they replied promptly that they will be happy to triple it for now, which should give us plenty of growing space. Hopefully that higher limit will kick in very soon; but until then we may be out during the final hours of the (California) day.
Not a bad problem to fix, though! Thanks for using our site so much guys :)