When the mid 2009 MBP 13" and 15" were originally released, the drive controller was set at SATA I speeds (1.5 Gbps). Apple received some bad press about it's new "Pro" machines being limited to SATA I speeds (making SSDs not run to anything close to their potential). Apple quickly released EFI 1.7 firmware upgrade that switched the drive controller to SATA II 3.0 Gbps. Although, after the firmware upgrade the controller could now operate at SATA II speeds, a component in the MBP does not allow the SATA II signal to function properly. Under EFI 1.6 SATA II drives would operate in SATA I mode, which was fine. Under EFI 1.7 SATA II drives operate in SATA II mode - which does not work. Like I said before, speculation is that the problem is actually the drive cable, as some have reported swapping the DVD drive cable with the hard drive cable and eliminating the problem. Apple also appears to put its own firmware on the drives that it puts in its machines - meaning that they are not exactly the same as the ones we can order. That being said, lots of people who are running stock hard drives with this machine are also reporting problems (those drives are probably really running in SATA II mode). The original drive that came with my MBP was a 160 GB SATA I drive which functioned flawlessly under EFI 1.7. My 320 GB SATA II drive would not boot after a shutdown, unless I would reset the PRAM - Every single time. Another solution would be to drop a larger SATA I drive in there, but they are hard to find, cost more, and don't usually come in capacities greater than 320 GB.
The best that I can say is that if you want to purchase one now, and don't want to worry about the EFI issue - just use the stock drive, and don't plan on upgrading it yourself. If you run into issues with the stock drive (which some do), at least Apple will take care of you in making sure that your machine runs correctly. If you live near an Apple store, and have an issue, you can just bring it in there and they might be able to help you. If you are going to get a MBP 15" in the base config with the 250 GB drive - I would bet it would be a SATA I drive, which means you would most likely not have any issues.