Well, effective this week we had one of our Scrum team members "borrowed" by another team for 6 weeks. (My manager insists that it was purely my decision, although it was one of those "either do this or come up with a better alternative" decisions. :-) ). But it was for the good of the overall team, so there's no question it was the right thing to do.
We were about a third of the way into sprint 5 when this happened, so there was definitely impact on this sprint. Fortunately the person who is out of the sprint didn't have any items that were key to the sprint goal, so we dropped some nonessential items (and will drop some more), and it appears that the sprint will still be viable (but it will be close).
The great thing is that the team took it in stride and was "self healing". It was clear which things had to get cut, and there wasn't much emotional wrangling or arguing over the cuts.
One of the nice things about Scrum is that the impact of this resource loss was "contained" -- it clearly affected sprint 5, and would impact the start of sprint 6 if sprint 6 occurred immediately after sprint 5. But people can grasp the impact in clear terms and don't seem to feel lots of Fear and Loathing about the impact.