Yup. Apparently he just decided not to show up for the first day of read-throughs. Caused a bunch of trouble since they suddenly had to re-write everything to fit in his leaving. I was pretty impressed with how they handled tonight's episode - that was a leftover from last season (delayed in respect of the VT shootings), which is why they had all the voiceovers of Gideon reading the letter. I believe he appears very, very briefly in next week's episode, and then that is it.
It makes me sad - I started watching the show because I loved MP in Chicago Hope. I have several of his CDs as well, and have been to a concert of his. I really want to like him, but he's apparently a bit of a difficult person. Oh, well. Even without him, CM is one of the best shows on TV.