Amazingly enough, most people who have put them near each other haven't complained much. I am going to assert that this will PROBABLY shorten the life of any HDD that has to deal with the vibration. I'm not saying it will fail right away, but you may not get your $150 worth if it dies in only a year or two.
I don't konw if there's a really decent answer to this right now. For constand vibration, I'd guess a set of gromets would dampen it really well, but that reduces the HDD's heat dissipation. Always a trade-off...
i really don't think it should be much of a concern. i haven't heard of any harddrive crashes due to the HD being next to subs. now of course it may shorten the lifespan a bit... now if it was me, i would not put the computer directly behind your subs - just to be safe. i would space the two at least a foot away from each other.
I've got two HDs in an old desktop case that is standing vertically on it's side. It's directly behind the back seat in the trunk with my subwoofer box right up against it (two 10"). So when you open the trunk you only see the sub-box and can't see the PC. To access the PC, you gotta fold down the backseat.
Anyway, I've been doing that for 8 months and have had no adverse effects from vibration or from the magnets. And since I'm using some old free hard drives that I don't care about, I've done absolutely no special mounting. They can crash and it'd be cake to replace the system with a new drive (which I would take more care in mounting). All of my MP3s are backed up at home, so I don't worry about losing those.