As others already wrote, vSphere USB Passthrough performance seems limited to about 6-7 MBytes.
I've done my own testing with HD Tune Pro 3.50 and a 500GB WD Elements USB 2.0 drive:
Physical Host, Windows 7 x64: 33/23 MByte/s R/W
VMware Fusion 3.1.3, Mac OS X 10.6.4 x86_64: 23/17 MByte/s R/W
vSphere ESXi 4.1.0, Windows XP x64 SP2 VM: 7/6 MByte/s R/W
Hardware used for testing was a 2009 Nehalem Mac Pro 8x2.26GHz Xeon E5520 w/HT and 16GB DDR3-ECC. Additionally I reverified the vSphere performance numbers on a whitebox system, Core 2 Duo E6400, 2.4GHz, 4GB DDR2 and got the same meager 6-7 MByte/s.
I also compared USB passthrough performance between WIndows 7 x64 and Windows XP Pro SP3 x86 on VMware Fusion, but saw little difference in performance. I've not yet tested with Win7 or W2K8 on vSphere, but I expect similar results.
If vMotion compatibility is enabled, datarates will be slightly lower and you'll see much higher cpu lood than in non-vMotion mode. A single VM seems to peg one core at 100% during benchmark in vMotion mode on the Core 2 Duo system, while non-vMotion was somewhere around 8% CPU load on that box.
@TrevorBell: I can't quite understand your claims of USB 1.0 vs. USB 2.0 device: An USB 1.1 device maxes out at 12 MBit/s, so you'd see maybe 1.5 MByte/s instead of 7-8 MByte/s. The numbers you posted above under "Update:" can probably be attributed to compression in the first run and to little change + changed block tracking in second pass. So I don't think you're getting more than 6-7 MByte/s throughput fromt he USB device. You could check with HD Tune Pro's Benchmark or Disk Monitor during backup, which shows troughput and IOPS.