According to Bryan Alvarez, the UFC has still not found an opponent for Cain Velasquez at UFC 90:

UFC sources say that despite reports of Cain Velasquez vs. Antoni Hardonk for the Chicago show, as of this morning, there was no opponent for Velasquez.

Dave Meltzer recently noted that they are having a lot of trouble finding people to take fights against Velasquez.  He is very good, but he has no name value yet, so fighters don't see any upside in facing him.

Fabricio Werdum, the number one contender, is supposedly booked on this show against Junior dos Santos.  This fight really makes no sense at all, it's a guy that is barely known to UFC fans against another that is completely unknown.  If Werdum is the top contender, he should fight Velasquez on this show.