I'm assuming its SATA3. I don't think it really matters, I think the important part is the speed on the chipset...
If a 5,400rpm or 7,200rpm HDD goes in there like a WD Scorpio Black 750gb, even SATA I would be more or less enough, as the speeds of 2.5" drives will never saturate SATA II and especially SATA II.
Look at these benchmarks:
http://www.hardwarecanucks.com/foru...pio-black-750gb-2-5-hard-drive-review-10.html
According to the benchmark, 126mb/s is the real world speed for that drive, SATA I @ 1.5Gbs maxes out at 187.5mb/s (1.5 gigabits = 187.5 megabytes).