I get the impression from the Defense Professor's dialog that it probably isn't permanent, but the way he hesitated before saying good day makes me wonder if even he is unsure about that after that last star spell.

I was also pleased to see that ZOMBIE! Quirrel still has the cognitive capacity necessary to crawl back toward the castle from the depths of the permitted forest. Although that's probably not that great; it'll probably suck to be the real Quirinus Quirrell very soon, and if he's had to be aware of all the further suckitude over the past year, that would probably have a net negative in utility.

For the past week or so, I've been occasionally imagining Harry running off after some important Voldemort event, and stumbling across the dead or dying Quirrell, just before coming across Voldy's new body, and being all concerned/confused in the few seconds it takes for him to figure out what's been going on. Considering that Quirrellmort is not an idiot, I kinda doubt this would play out like this (I'm imagining it at a graveyard, even), but it seems like we're close to the real Quirrell making some appearances, such as the one in this chapter.