@Michael Seeker: There was no more formal agreement with the prospective tenant other than me saying "Hey, I'm supposed to be buying a house soon and I know you need one soon. Wanna come by and take a look after I close?" Normally I'd agree that the later closing date wasn't the seller's problem but this wasn't an "on or about" situation. This was all parts in place, everything ready to go, last minute pulling-of-the-plug thing.
@Jason Hirko: The algebra of this is exactly what I was looking for. What are some X, Y, and Z specifics? I could try to get some of the closing costs covered or part of my lawyers fee maybe. The fact is I'm not interested in walking away from this deal and I've never been good at hardball.
@Russell Brazil: I am as used to closings moving as the next guy, just never this late. My realtor says this is a new experience for her and she's been in the business over twenty years. My lawyer tells me they didn't put a contingency in the agreement for this kind of thing and so I may be able to force them to close or to pay me rent until they actually finish moving out.
I appreciate all the responses. I've been told that I'm "too nice" when negotiating and while it is still making me money to act this way I start to wonder if I'm being taken advantage of on some occasions. Either way I'll end up with a very nice cash-flowing property at a good price.