The rumour is true: you will be able to drive V8 Supercars in Forza Motorsport 3. Eight of them, in fact; evenly split between Holden and Ford.
Xbox and V8 Supercars Australia have announced the partnership that sees eight cars from this season's championship series appear in the 360-exclusive racing sim. They are:
Holden #2 Toll Holden Racing Team Commodore VE #33 Garry Rogers Motorsport Commodore VE #39 Supercheap Auto Racing Commodore VE #51 Sprint Gas Racing Commodore VE
Ford #1 TeamVodafone FG Falcon #5 Ford Performance Racing FG Falcon #9 Stone Brothers Racing FG Falcon #25 Britek Motorsport FG Falcon
However, there will be no Australian tracks to race them on. Instead you'll be taking your chosen V8 to "world-famous tracks, including Suzuka, Nürburgring Nordschleife and the Sebring International Raceway, as well as new environments like the mountainous Montserrat region in Spain, the rugged Amalfi Coast in Italy and the American Southwest."
It's no substitute for the lack of a real V8 Supercars game (hey, Codies, what's going on there?), but eight cars is definitely better than none. Check 'em out.