Most HDD's are certainly too slow for USB 3.0, or for that matter for SATA 2.0 interfaces. The SSDs are not and while they are relatively expensive at the current time, it'll change with time.
There are fast USB 3.0 external drives available now for about 200 bucks, such as these:
Newegg.com - OCZ Enyo OCZSSDU3-1ENY64G 64GB USB 3.0 MLC External Solid State Drive (SSD)
Newegg.com - Kingston HyperX MAX 3.0 64GB External USB 3.0 Flash Drive Model SHX100U3/64G
Both of them built utilizing a SATA II SSD drive and bridge chip to convert between the SATA II and the USB 3.0. The speed difference is mainly due to the type of chip used in the two devices that translates/encapsulate to/from USB and SATA . The extra steps and processing will slow down the effective throughput without question. It seems that Enyo does have a faster conversion chip than the HyperX.
But the main point is that on the back end, it is still SATA II storage with the USB 3.0 external storage devices. With this setup it is impossible to make the USB 3.0 faster than SATA II, or eSATA II. As long as the USB 3.0 and later will rely on the SATA x back end, it'll be impossible for it to surpass the utilized SATA x throughput.
Someone actually took apart the USB 3.0 SSDs:
OCZ Enyo Review
reghardware: From the lab to the mobile living room
I have to admit that, regardless of the throughput loss, the Enyo does look cool
It's tempting, especially in light of its USB 2.0 support, but it's too early to get one. There aren't many systems around where the USB 3.0 interface can be used and it's way too expensive for a USB 2.0 external storage.