Previous reports of USB 2.0 chipsets built into PowerMacs led to some conflicting reports regarding the activation of this hidden functionality through experimental drivers.
Recent (cryptic) reports suggest that the iMac may contain the same functionality and will receive an unexpected (but official) update from Apple this Wednesday to enable this functionality.
Correction: Well, looks like the reports were too cryptic. :) The update on Wednesday will simply be a security update -- while future iMacs will get USB 2.0.