danb35:
Found it:
http://seeedstudio.com/forum/viewtopic.php?f=9&t=2416&sid=da67ed081438154a131067df942a4e59
Interesting. I've been playing with eagle and carefully measuring my Diecimila and some shields I had made using the 10x10 service. It looks like the headers are 1.9" apart and 50mm is 1.968 inches, so I should have 0.8mm minus 1/2 the drill size on the outside of my header (mixing metric and imperial is not fun). The standard square posts in headers are 25mil, or 0.635mm, so I don't think I should use a drill size smaller than 0.8mm.
That actually does let me have some board material left around the hole, thought it's cutting it awfully close and in your image it looks like he as a lot more than my estimate.
It's definitely worth giving it a try next time I need a shield, because as I said even if I end up with my holes split, the board will still work.