Actually, it CAN damage your hard-drive, and it can do it in two ways; physically and through software corruption. Hibernate is when the computer takes everything in RAM and writes it to the hard-drive and immediately powers down, leaving the read/write head where it left off so that when you power back up it can begin reading immediately. The rate at which the computer writes the RAM contents to the hard-drive sometimes leads to errors (kind of like burning a CD at slower speeds to ensure accuracy vs. getting errors if you burn at a faster speed).
Over time, these errors can add up and confuse your system to the point of getting the dreaded BSOD. The second way it can damage the hard-drive is physically. Hard-drives are like old-school record players. They have a magnetic platter, which is where everything is stored (kinda like a record), and a read/write head (which is kind of like a needle, except this needle isn't supposed to touch the record). When you power down a computer normally, the read/write head goes back to it's home position and is "locked" in place. Kind of like when a needle gets to the end of the record it raises up and goes back to home position.
When you put a computer in hibernate, the read/write head stays out over the platter (so it can begin reading immediately as earlier stated). The problem is, when it's over the platter, it isn't "locked" down. It can float around. And it takes very little bumping or jarring of the computer (especially easy to do with a laptop) to make that read/write head actually hit the platter, causing scratches. Well, like the old record player, if you run the needle across the record, your record is screwed. It's the same with your hard-drive. I see it happen all the time.
College students hibernate their computers and dump them in their laptop case and go riding off across campus on their bikes blissfully unaware of the potential for screwing up hardware seriously. My advice, if your computer is running cleanly it should only take about 45 seconds or less to boot up anyway. Spend the extra 20 or so seconds you have to wait over the time it takes to boot up from hibernate and save yourself a few dollars on replacing your hard-drive and losing your data later.