Usually I agree with FSP, but I've got a problem with her post from Monday. That is, when she says that if a grad student thinks that an advisor may be leaving,
"...you could ask your adviser a direct question about it. Either you will get a non-answer, in which case you should respect the fact that you don't have a right to know everything about your adviser's professional decisions despite the fact that they affect you, or you will get some information that will either be comforting or disconcerting."
Here's my issue, and it's one that several comments hammered away at: having your advisor leave is a really, really big deal. When a grad student's advisor leaves, it's a lot more than disconcerting. A grad student may have moved a significant distance, may be several years into a degree, and may have their entire income (such as it is) evaporate in the institutional confusion.
I'm not saying that professors shouldn't be allowed to move to greener pastures. But at least they should have the decency to make sure that they don't leave their students high and dry.