I ran into a funny issue similar to what @Harjit described. The hotel I wanted wasn't listed as sold out, but when I went to make a reservation it told me it was unavailable. After a bit of clicking around I noticed that if I changed the check-out to Thursday (instead of Friday), it let me make a reservation. Then I just booked Thursday night ONLY at the same hotel through my company's travel system (and got a cheaper rate than what was available during the week).
So now I have two reservation numbers for the same trip, but hey, it worked.