AFAIK, using my limited knowledge of physics...putting a DVD/CD/Hard Drive on its side will not hurt it. Think of the spinning disks as gyroscopes.
Gyroscopes are stable as long as their axis are not tilted while the gyro is in motion (ie: spinning). If you've ever used a gyro you will notice that while holding the gyro still there is no force on the axis except for the outward force generated perpendicular to the axis by the spinning disk. However, once you try to tilt the gyroscope you can feel the force that the spin is causing.
Likewise, a DVD/CD/Hard Drive should be okay no matter how it is situated, as long as you do not move the unit while the disk (or disc, however you like to spell it) is spinning. You'll notice that the XBOX manual instructs you not to move the XBOX once it has been powered on...I assume that it is for this reason.
To strengthen my claim....we have servers at work that have both their hard drives and CDROMS mounted vertically that have been running for years without problems. If it were indeed true that mounting these devices vertically was bad then we should have a higher failure rate in these devices.
If anyone can confirm or disprove my theory then I encourage them to do so.
Damn, that's deep...