Ummm...i just spent a ******** on this new 2.53 ghz 13''...I'm not sure im understanding this...what does it mean?
Not much. They are talking about the speed at which the hard drive can send data through the connector cable to the computer. 3.0GBit SATA can send 300 Megabyte per second, 1.5GBit SATA can send 150 Megabyte per second.
However, that is the speed at which data can be sent through the cable. The hard drive cannot actually read data from the disk at that speed. That is at the moment limited to maybe 80 or 90 Megabyte per second. So at the moment, 1.5 GBit SATA vs. 3.0 GBit doesn't make much difference for a laptop. It would make a difference if you connect a RAID drive to a MacPro that can exceed the 150 Megabyte per second limit of 1.5 GBit SATA.
For comparison: Firewire 400 has a maximum of 50 Megabyte per second, Firewire 800 has a limit of 100 Megabyte per second. So Firewire 400 _does_ actually slow a fast external hard drive down.