USB is aimed at external and portable devices, neither apply very well to solid state drives. USB is also a high-latency, high-throughput interface, which is nice for mass storage but not for a system disk which is the prime usage for SSDs.
So probably not, although some SSDs do have a USB 2.0 port like some OCZ products. The latencies will nullify alot of performance potential of the SSD.
While everyone knows throughput, expressed in MB/s, actual performance is always expressed in latency. Though both are related, latencies are more difficult to understand, and companies don't care about correctness but want to brag with high numbers; not low numbers. So latencies are forgotten, but are actually paramount in virtually all computer...