They are, after all, the same operating system.
Not necessarily. IIRC, there are two versions; NTSC, and PAL. If you have the PAL PS3, then you'll be able to play games petty much fine, however you will also need the PAL correct TV. Most games are region free, however I will do a bit more research onto it. It will probably not display NTSC movies well though, but again I'll look into it…
EDIT: Assumptions seem correct–you can play PAL and NTSC games on a PS3, since they are region free. However, movies aren't, sadly.
EDIT PART DEUX: The only thing that will suck is that if you have a collection of NTSC games from the PS2 and PSuno they won't be backwards compatible with you PS3.
EDIT TRES: As long as you TV supports the FPS needed. So if you have a PAL game and a NTSC PS3 you will need HDTV (Component or HDMI) to play the title. Vice versa as well. Just make sure your TV has Progressive scanning in either HDMI or component, with either one of those.