The theoretical max of USB 2.0 is 60MB a second. I have occasionally exceeded 40MB a second. I think it was Jamesh who tested the Alpha board at like 12MB a second. This about right considering the power of the processor involved. I see only two ways that you could possibly run USB faster on the RasPi, and both would triple the price to gain faster USB 2.0. One is to use a SOC with a faster ARM. The other is to install a dedicated bus mastering chipset. There are other SOC"s that could be used such as the OMAP"s from TI, but you would have to start development from scratch. I also believe the cost of the OMAP"s per unit is higher. I have not seen any tests of the OMAP that actually state what their transfer speed is through USB 2.0. As to using a chipset to speed up the USB, it would not require a complete restart but it would probably be June or July before new boards would be ready for sale, at triple the price and double the size. Just to get faster USB 2.0.
That problem only gets worse when you try for USB 3.0! An SOC is not a single component as it looks from the outside. Inside it has a:
These are just the major components and there many more inside. As JamesH said it would cost millions to build a new SOC. The BCM11311 has VideocoreIV and a dual core processor, but it still has only USB 2.0. I am sure this SOC costs significantly more than the BCM2835. USB 3.0 theoretical max is 600MB second. The real world transfer would be around 300MB a second, though an ARM based system may not be able to get this out of it. If it was just the same percentage as the RasPi it would be around 100MB a second which would be a significant gain in Data performance.
If ARM based small computing devices and home electronics based on ARM technology really catch on I hope that USB 3.0 will eventually be incorporated into a SOC. I would be happy to see Ethernet incorporated into the SOC instead of needing a external chipset to get it, and they are out there. The market will determine what is worth designing and manufacturing. RasPi could set those wheels in motion.
The future will tell. Before we get too far into what we would like to see, let"s see the RasPi in full production!