I may be anal here, but the bid should include the signature of my initial message, so that we know which auction we're talking about.
The date is in the signature, so there's not much chance of confusion...
I'm not sure the timestamping in a GnuPG signature is trustworthy. If it was, the bitcoin network would use it instead of a distributed time server.
Anyway, now that I think about it, signing bids is not necessary. It is really an auction protocol issue. Here the proof of concept is about bonds, not forum auctions.
What has to be signed is my promess to pay. So once I get a bitcoin address from the auction winner, I'll sign it along with the bonds terms, and that will be all.