I'm thinking about getting a larger drive, too; I checked some reviews and there seems to be noise about a "known incompatibility" with MBP and these drives.
This was the description given by one reviewer:
The WD drive has a KNOWN compatibility issue with Apple's Macbook Pro line. The drive constantly attempts to park the needle and the Macbook will IMMEDIATELY start the drive back up. This is irrespective of the "Put the hard disk(s) to sleep when possible" setting. This constant back and forth causes the drive to cycle up and down in a way that will impact performance in the immediate term, and shorten the life of the drive in the long term. Unfortunately many people don't take a noticeable performance hit, so they don't notice the constant cycling of the drive until it starts to click after a few months.
Anyone on these forums experience this issue? Is this related to the issue with the EFI firmware update?