So yes, most of the ISV's apps works correctly.
For Microsoft ones, it's kind a lottery. Mostly not because of the app itself, but because of crappy restrictions you put on the installer. That's kind a shame, isn't it ?
Simplest example : Windows Live. It's now impossible to get MSN Messenger or any of those Live Apps installed on Win XP 64. Not that the apps won't work, just that the installer will detect XP x64 and refuse to continue.
WHY ? Why do you push ISVs to go 64 if you restrict your own apps ?