With full drivers, including ATI, and dynamic non-destructive partitioning.
This is the best thing Apple have done for a long, long time and it's going to hopefully be huge. Now people can use OS X for normal work, and dual-boot into Windows whenever they need to run Windows-only engineering software, AutoCAD or Half-Life 2. I don't own a Mac, but I've always liked OS X and their laptops and now they're definitely on the purchase agenda.