Most of the rides are open - certainly Soarin' is. The only "big" attraction that isn't open is Grizzly River Run. Everything in Hollywood should be open except for the Aladdin and Playhouse Disney shows, and most of the rides in Paradise Pier are open - Golden Zephyr might not be, but Screamin', Mulholland Madness, Sun Wheel, and Carousel are. All of the kiddie rides in "a bug's land" are open, too. Soarin' always seems to have the longest line - most everything else is under 10 minutes.
We've gone the last two years and had a good time - it's fun seeing people in costume, and the Disney characters are in costume, too. LOTS of trick-or-treat stops, with plenty of candy, and the lines usually aren't too long for those.
There's some additional information and reports on the events from last year's party on AllEarsNet.com:
http://allearsnet.com/dlr/tp/dca/mht.htm