Thanks for the response, and sorry if I was unclear; I thought it would be obvious that I wasn't asking about a 6Gb HDD (Does such a beast even exist?), but about 3Gb/sec v 6Gb/sec Link Speed.
At any rate, though, I've decided against all the 6Gb/sec WD drives I was looking at in favor of
this well-reviewed(*) Hitachi drive (despite the fact that it's manufactured by WD), because the Hitachis I've had have been trouble-free, as opposed to the OEM Toshiba that came with my MBP and the WD Scorpio blue I purchased, both of which tanked with bad blocks in the last two weeks. (*) More good reviews
here.
Edit: Any opinions or stories about the drive before I buy it?
And getting back to 3Gb/sec v 6Gb/sec, does that work similarly to FireWire 400 v 800 and USB 2 v 3, with the 6Gb drive being compatible, but minimized by the 3Gb limit of the computer, or is the 6Gb/sec drive simply incompatible?