Between Blizzard, Valve, MySpace/FaceBooK/WebkinZ/and Disney the computer along with a mouse and keyboard aren't going anywhere. So, why hasn't the mouse invaded a console? As far as I'm aware Wii/360/PS3 all have keyboards. I believe the PS3 can use a mouse on certain games(I only know of UT3).
If you have a controller, wand thing, video record thing, arcade stick, racing wheel, music controllers, keyboard, and all these other devices, is it really that difficult to include a mouse? It cannot be that much extra programming.
A mouse has extreme speed and extreme accuracy, making it ideal for pointing and clicking. It's unmatched by any control stick/d-pad ever made. Forcing controllers to be synonymous with "n00b" or auto-aim.
Because it has few buttons, it's ideal for any age group and is quite easy to pickup and play for the most part.
A lot of issues come to play with a mouse and keyboard. You need space to sit and enough space for a mousepad. Extra pads help too. The mouse easily gets dirty. I haven't used a wireless mouse in years, so, I'm gonna go ahead and say it still needs to be corded.
A single handed controller could be made to go along with a mouse.
FPS'ers alone sell in the millions. Golden Eye(Eight million+), Halo(Eight million+), Half-Life(Eight million+). A mouse and keyboard will sell. It'll sell even more when users are repeatedly killed by a mouse/keyboard player.