I can't imagine that it's bad form to attack in this situation. There were still around 90 riders back with the Mayo group and he had his team, plus there was around 60 km to the finish and there was a breakaway up the road.
It goes to show what a benefit a strong team is. If the same thing had happened to LA, Ullrich or Hamilton you know that Postal, T-Mobile or Phonak would ride all out ala a TTT, if necessary, to catch up (and probably would) or at least limit the loss to a manageable time.
Unfortunately, Euskatel couldn't do it and the lack of a strong overall team probably cost Mayo a chance of challenging on GC. Really too bad on day 3, but you almost can't win anymore without a strong team.