There was, I believe, a thread where this was discussed in some detail a while back when Apple added the magnetic remote holder to the previous-style iMacs, but I can't seem to find it now.
Basically, though, magnetic storage media (floppies and HDs), by definition, relies on small changes in magnetic field to store data, so they don't particularly like large magnetic fields--have a tendency to corrupt data stored. CRTs also dislike magnets, in that you can screw up the beam in a permanent way under some circumstances (I did this as a child once).
Most everything else in a computer actually doesn't much care about a magnet. And in reality, even hard drives are relatively well shielded from magnetic fields, so they're nowhere near as sensitive as the image I had drilled into my head as a child. Plus magnetic field decreases as an inverse cube, which is to say that it falls off VERY quickly with distance.
Bottom line being actually computers aren't THAT sensitive to magnets, it's just a whole lot easier to teach people "magnets + computers = bad" than "well, it's generally ok, but if you hold one too close to a drive or move it in a certain direction you could cause serious data corruption, particularly if it's not grounded..." etc.
So be sensibly careful, but don't freak out about it; the hard drive is probably far enough inside and well enough shielded that even if you were to hold one of those magnetic blocks right against the case there it wouldn't affect it.
