On PS platforms, it's totally up to the soft developers if they want to include M/KB support.
The PS3 hardware itself supports all standard USB peripherals, including any mouse and keyboard.
For FPS games or RTS games (not a strong genre on consoles anyway), M/KB players would have a user interface advantage, meaning developers would have to add a check option for players to exclude M/KB users on their server, severely limiting the number of available opponents for M/KB users.
Not a perfect solution, but the option is there. It just seems that there isn't a huge demand for M/KB control options on consoles (other than for those who need that crutch and can't play with a control pad), even for chat based online services like PSN Home, where the vast majority of all users online are typing... very... slowly... with... a... control... pad...
The main reason is the way most consoles are set up: to a TV set back from the couch. It's just not practical to use a mouse and keyboard if you're not sitting at a desk.
This is coming from someone who has a gaming PC set up to a big screen flat panel in the living room; it's not the most practical solution either, which is why I typically find myself gaming on my productivity PC instead.