Yes. I was able to install two SAMSUNG Spinpoint M8 HN-M101MBB 1TB 5400 RPM 8MB Cache 2.5" SATA 3.0Gb/s Internal Notebook Hard Drive -Bare Drive from newegg.com in place of the single 500G drive Apple supplied.
I can confirm I was able to install a 2nd drive in mine just fine. In fact I replaced the stock 500G drive and installed two SAMSUNG Spinpoint M8 HN-M101MBB 1TB 5400 RPM 8MB Cache 2.5" SATA 3.0Gb/s Internal Notebook Hard Drive -Bare Drive from newegg.com.
I replaced my stock 500G drive with two 1T Samsung drives, been running almost a week now, 24hrs a day, non-stop. I've been copying data back and forth from both drives. I haven't noticed any odd fan behavior. It does kick in when the system is under a load, but nothing I felt was in-appropriate. As soon as the system cooled off the fan slowed down. I've also been monitoring the temps with iStat and don't see it getting very hot. Overall I'm very happy with my purchase and cheap and easy upgrade to a 2T system over Apple's prices. The kit was very easy to install and the documentation is pretty clear and understandable. Even if you're not technically inclined but not afraid to get your hands "dirty" you should be able to do this upgrade.