From PCWorld:

The third-generation Universal Serial Bus interconnect will transfer data at speeds up to 4.8Gbit/s, ten times faster than USB 2.0’s 480MBit/s. It will be backwards-compatible with USB 2.0, which is backwards-compatible with the first USB 1.1 definition.

That is how you make a specification – without caveats, and without breaking backwards compatibility. This gives consumers the choice to opt in, as opposed to AGP / PCI-E.

Now if they can just make some identifier on the cable that lets me know which way to plug it in…