So, I seem to have stumbled upon a mildly nasty and probably-unimportant-to-most-people 'bug.' I don't have many other DAWs to test with, but if I'm outputting a dense stream of MIDI CC commands (say about 1500 per second) on one of the 'From MaxMSP' busses, and into Pro Tools 9 - Pro Tools will become completely unresponsive to user interface elements (mouse or keyboard have no effect on anything). It will stay unresponsive until you stop the midi coming out of Max, and even then it will take upwards of 15-30 seconds to 'catch up.' I've noticed no issues if I use the IAC bus to get the CC stream from Max to Pro Tools, so there's a nice little work around there. But I have no idea why there's the hangup with the MaxMSP busses compared to the IAC bus.
For anyone that happens to have a copy of Pro Tools 9, I'm including a little patch that can (somewhat) consistently cause this problem on my machine, the specs of which follow:
OSX 10.6.6, 8 Core Mac Pro (Xenon, I believe) 3ghz. 5gb RAM, MaxMSP 5.1.7 (going to check 5.1.8 in a moment, but I didn't see anything related to this in the change log), Pro Tools 9.0.0, 9.0.2, and 9.0.3 tested. Other equipment that might be related (IE, midi stuff hooked up): NI Maschine, Casio PX-130 Digital Piano, Aleisis Q25, Akai LPD8, and a BCF2000.
Max's DSP settings don't appear to matter much, I've tried IO/Signal vector sizes ranging from 32 to 2048, with and without overdrive/audio interrupt, to no avail.
Another fun thing to try is to run Max as a Rewire slave, and use a rewire midi bus - then Pro Tools will just crash without warning, at random! I blame that on Avid more than anything else, though.
If anyone else can or can't reproduce this issue, I'd be interested to know. It took me about a day and a half or so to narrow this issue down to the MaxMSP virtual midi busses, and maybe it can be narrowed down further.