[quote name='force_assassin']I've heard that vertical standings can wear out the drive faster (since gravity constantly wants to pull the disc down - and it will be pulling on side of the disc instead of the larger even flat area in a horizontal positioned system)...[/quote]From my basic understanding of physical mechanics and the assumption that media discs these days are fairly uniform, I'm not sure I'm buying that. That claim might have something to do with how the drive laser moves, but I'm just speculating, here.
[quote name='force_assassin']...and possibly damage your system if it is bumped while it is on and a disc is spinning.[/quote]That is absolutely true, but I don't know if it is more of a problem when the system is vertical rather than horizontal.
That said, my Wii, 360, and PS3 are all presently horizontal for safety reasons. They all seem to get enough air flow, so I probably won't be going vertical anytime soon. Probably the only one I'd consider placing vertically is the PS3, since that thing is pretty huge.
If you wanted to go vertical with your PS3, I can't imagine why it would be a problem.