Take it to a car repair place. These days they can vacuum suck out dents like that and return it to new.
I like this idea.
Anyway, the LBS said they were pretty sure it was OK, but obviously wouldn't 100% commit.
I did the 60 miler on Sunday and it was absolutely fine. (well, the bike was - I was completely cattle trucked!)
I love steel even more than I did before!