When I loaded the project into 9.1 today and hit play, the mix was whacky, on every song/chunk. The instruments and vocals were all there, but they were not at all the way I left them (guitar too loud, can barely hear the vocal, but all of its echo and reverb is there, etc.)
I knew I didn't leave that project that way because I have reference mixes. I got the distinct impression that plugins might be responsible, because of the symptoms. I don't know which ones per se, but I used a lot of Slate VMR and Soundtoys in that project.
So I reinstalled 9.02 (I didn't think to rename it before I installed 9.1). Pulled it up, and all was well again, sounds the way it should.
Serious drag for me, because I was really digging the new audio and VI performance enhancements in 9.1. I suppose I could use both versions of DP, just using 9.1 for the new stuff.. but that might be more trouble than it's worth. Guess I gotta go back.

I had some other (much older) projects that would load and immediately crash DP 9.1, maybe I'll try these as well with 9.02 and report back..
EDIT: It was the Slate Virtual Mix Rack that was the problem. Toggling bypass on the plugin fixes it. I'm on the newest VMR, version 1.6.3.1
I do not Believe DP 9.1 (in and of itself) is responsible for this issue.