i know quite a few people who kept their PS2 vertical and it eventually skrewd their games and dvds up a bit. the disc reading laser i think is not meant to move on its side. and the disc tray isn't REALLY meant for that either. because a mechanism inside the console around the laser has to lift the disk (by the center hole) off the tray in order to read the whole disk, having it vertical means gravity will make it hang down and rub on the inside lip of the disk tray, eventually stratching the disk.

im not making this up it happened to three friends of mine so i dont think its a freak coincedence. it might be though!!??

i put mine on its side for a bit and it started to make a funny stratchy noise when it loaded up a disk, so i put it back on the bottom and it stopped right away.
well thats my thoughts anyway.

also i put my tv on a cabinet so there is perfect room for my PS2 horizontaly. And my PS2 ALWAYS comes first!!!!!