If you're attacking, it's the building on the left-hand side of stage 1 cap 2. They've removed one of the doorways. Spies and some pyros are annoyed, as are scouts who used to be able to go out that door to get behind the cart if it was up far enough.
I, on the other hand, think that it's great. In my experience, it really keeps the red team out of that building, and gives the blue team someplace to regroup and prepare for a push. I like the change.
I just wish valve would have included a blurb in their changelog about it.