I really dont' see HP taking any audience AWAY from Rent. Sure it'll do better the box office, but HP's a big global phenomenon (sp?) and any movie would have a hard time unless it was also a big, family based subject matter with a giant following.
Personally, and I actually just planned this tonight, the group I'm going to the midnight screening of Harry Potter with just made plans to call around for a midnight (crossin' our fingers) showing of Rent so we can go to that together too. Everyone I know is going to both, so...only difference for me is I wouldn't take my lil sis to Rent, but I'll prolly take her to see HP.
The ADVANTAGE is that if you have people who are just wanting to catch a movie on a Friday night, they are definitely more llikely to see Rent if they HAVEN'T seen the first 3 HP flicks or read any of the books. No one's going to just go catch...well, not even a sequel - the 4th installment of a movie they haven't been following, so there's always that advantage...
I don't think it'll take any audience away from Rent that it would have gotten in the first place, y'know?
If I show you the darkness I hold inside, will you bring me to light?