The Joan Rivers episode... it kind of annoyed me at first, because the episodes are all named after the patients, and I thought "oh, if I hadn't known Joan Rivers was in this episode, it would have been such an amazing surprise."  Then Alec Baldwin comes out of nowhere and I'm floored, that was so awesome.  Although part of me couldn't help but think back to his "bad soap opera doctor" character on SNL, who boasted about doing all this medical research but pronounced every single technical term wrong (anal cay-nal etc).

I also loved that Bobolit's assistant when he's Blue Velvet-ing is Karen Muriyama, a staple from the UK Whose Line (she was also the fake southern maid in "The Campaign").  I'm sure there's TONS of other great casting bits I'm forgetting about, but I love that the show was getting big enough to attract all this awesome talent (I KNOW I've seen Ava's son somewhere but I couldn't place him for the life of me).