I have Rex as my companion currently (Boone called it quits at the slave camp) and have found that after taking the raft to Caesar's fort that Rex doesn't spawn inside the fort. However, when you try to enter Caesar's tent it will tell you that you must enter alone, even though Rex isn't accessible. I've gotten around this by telling Rex to wait in Cottonwood Cove, and it will now allow me in to Caesar's tent. Here's to hoping I can pick Rex up on the way out.

I was able to get Rex back after returning to Cottonwood Cove (where I left him) after my business in Caesar's fort.

Mind you, my time in Caesar's tent wasn't without some glitches: http://www.youtube.com/watch?v=0Suox8scFLg

Wow is this on the PS3 version??