"Players can go online to vie with as many as five others (six in total); the vehicle roster is currently swelling to more than 500 cars, which range from the earliest race cars ever made to the fastest, newest games on the planet; it offers new circuits, many of which are based on real world circuits from across the world; and last but not least, it will feature better, more human AI...

"...The 500-plus car roster includes a glut of licensed cars (one of the series' biggest and best trademarks), and we've got confirmation on many of them (see the list below); but vehicles such as Ferraris, Lamborghinis and other European streets cars have yet to be confirmed for GT4...

"...While we played on three courses at E3 -- The Grand Canyon (rally), NY, and Tsukuba Circuit -- we expect dozens more tracks to appear. Since there were about 30 vehicles available in the E3 demo that means at least 470 more are to come. Weeee!!!"

http://ps2.ign.com/articles/429/429322p1.html