It has AAx4 enabled. I can only see some minor jaggies.
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The problem with SDTV will be in Revolution and PS3 too. You don't have pixels enough to put all the details that are designed in the game wich such a low resolution. With a 1080p downsampled to 480p you will loss MASSIVE detail and pixels too.
This isn't a Xbox 360 problem, is a generalizated problem. On NBA 2K6 for example, you won't see a detailed face on the players, if you only have 100 pixels for its representation... but in HD resolution, if you have 400 pixels you can appreciate the difference with the last generation. Of course, in a SDTV on the replays when you have tons of pixels for represent one or two players on the screen you will see a clear diference.
For example, in PGR3, even in 480 resolution, wou can see a MASSIVE IMPROVEMENT on the environment (textures, lighting, tons of polygons, etc). In the car, with only ~40K pixels for the representation you can only see minor improvements like the textures on the lights, on the Nissan logo, more detailed interior and inprovements on the lighting and reflecions.
But with that resolution you miss detaills like SKYLYNE Normal Mapped logo, heat distorision on the exhaust and other things that clearly makes this game a generation above PGR2.
http://images.xboxyde.com/gallery/public/2233/726_0005.jpg
And this will happen in almost every genre and game on the next generation, and with all the platforms.