Guys, don't be silly. The PS3 will NOT RUN OS X.
While the Cell is the perfect multimedia processor (HD video decoding and video editting and photoshop) it's terrible for general purpose stuff (it doesn't even support out of order execution).
Apple would not come out with OS X on the PS3 for one simple reason: Intel. PS3 is coming out next year, around the same time Apple is switching to Intel. The PS3 will be around the next 5-10 years. Does Apple really want to keep supporting a PowerPC system that long?
On top of that, normal Mac apps cannot use the Cell's sub-processor, and those can only use the primary processor, which SUCKS terribly on its own. So you'd have to buy a special PS3-edition of Final Cut, a PS3 edition of Photoshop, etc.
Sony was saying it was capable of running Windows or Tiger. That doesn't mean its going to. Getting them to run on the Cell would either kill compatability with normal apps (Windows binaries don't run on PowerPC), or kill their performance (Mac apps wouldn't use the SPE's and therefore run terribly slow).
It WILL run Linux however. Screenshot:
Frankly, this is awesome. Sony has stated they're supporting homebrew; they want developers at home (using Linux) to create their own games and applications optimized specificly for the PS3. I can imagine that if they take some open source video editting software and optimize it for Cell they could even pull in the video edittors crowd as it would have astounding performance; I remember the demo of the PS3 playing 12 HD video streams at once. That was unbelievable because my computer barely handles one.
The possibilities are endless.