As I said in response to a comment upthread, the existence of an implementation by someone who had no knowledge of the patent should be prima facie evidence that the idea is obvious to an expert in the field. That should get the patent invalidated.
The problem there is that you're trying to prove a negative, which is probably even harder than proving a patent is obvious.
Say Johnny Evilguy digs up your widget from the USPTO database from a Tor connection and then re-implements it. Poof, your patent is dead, and you can't prove he read it.
Karunamon is right - it would be virtually impossible to prove the second implementer never looked at the patent database.
Beyond that, over time things that were novel are no longer novel as the direction of technical development changes, so the timing would be important as well.
