FOUND A FIX to Acrobat closing automatically...
(at least, it's a fix for me)

I have Vista Ultimate 64-bit, and was running Acrobat Pro 8.0 for a few weeks with no problems. Then, it started closing automatically after approx 10 seconds. Prior to this I had disabled the service "FLEXnet Licensing Service" since it was in charge of FNPLicensingService in the process list. It bothers me that an activation service runs in the background all the time. Does it REALLY have to? No, okay?
Anyways, I disabled it and this issue occurs. Now I re-enable the service and start it, and Voila! Acrobat stays open.

If this is the fix for everyone and Adobe couldn't suggest this fix, then I'm "surprised" (being really polite, since this is a public forum).
I hope this works for everyone.
-Mike