This
Crave article tells of USB 3.0, which they report should show up in systems in 2009 or 2010.
Since it will include Fiber as well as the traditional copper, it will be backwards compatible with USB 2.0, but will also be able to speed data along at a theoretical 4.8Gbps.
Keep in mind this is theoretical. (see
this article about why USB 2.0 at 480Mbps is theoritically faster than Firewire at 400Mbps, but in reality doesn't tend to perform that way)
Your computer has to do all the work in managing that transfer, so while your system might be busy keeping a USB 2.0 device fed, it will have to be many times busier to keep a USB 3.0 device fed.
Systems are not going to increase their overall speeds by a factor of 10 in the next two years, so expect to see system performance drop as a result.
On the plus side, while your system is working harder to move that data, it will move faster, so you won't have an impact for quite as long.
We've already got Network cards offloading processing for various networking related tasks, how much longer will it be
before someone builds a USB co-processor? (which would kind of make it like the firewire devices... a processor for each device, it just happens to be on the other end of the cable)