You can't have Le Mans without GT race cars and prototypes. You can't have the Grand Prix of Monaco without F1 cars. You CERTAINLY can't have the Indianapolis Motor Speedway without Indy cars. And let's not forget that you can't have racing at Indy without INDY cars. So asking if Indy cars (or even the former Champ cars) is a VERY viable question. And if you ask me (and if PD is smart), there is a very strong likelyhood we'll see Indy cars in GT5. Believe me... I've wanted to see some more types of Formula cars, especially Indy and Champ cars. I mentioned this in my Blogspot blog entry on the basis of if Indy Cars are... not so much will happen, but COULD happen. So yes, I'd be VERY excited for the possibility of IndyCar Series machines. I am even excited at the notion of perhaps the old Champ Cars. If we could be humbled with the powerful Group B rally cars, if we could enjoy the amazing GTP race cars of the '80s and '90s, then why not some of the classic Champ Car machines? I wouldn't mind racing any Target Chip Ganassi cars like from Alessandro Zanardi, 1996 champ Jimmy Vasser, 1999 champ Juan Pablo Montoya... perhaps even Paul Tracy's Forsythe car in 2003 that won the title... don't forget about the option tires and the "push to pass" deal.
I'm looking beyond Indy Cars, but I'm very positive that we COULD see Indy cars for GT5. I'd say there's an 85% chance right now we'll see Indy Cars for GT5.