Well the theoretical max on USB2 should be 60MB/sec, so it's definitely a bottleneck for the fastest HDs (remember that even a drive with an average speed of 60MB/sec on SATA will likely have a higher peak speed, which would be limited by USB2, lowering the average).
As for the USB1.1 compatibility issue, it wouldn't surprise me if some of the first boards come with a mix of USB2 and USB3 ports. There's bound to be people out there with USB1.1 devices e.g. printers.