Say I wanted to paint the case of my Mac.....

I assume I'd need to strip it off all the plastic casing, and soak it in hot soapy water?

disassemble, sand lightly with really fine grit sand paper. the sand lightly again with really really really fine grit sandpaper. Then wipe with a damp rag, and allow to dry. Then paint.

What type of paint would I need to use - would spray paint do?

You CANNOT just use any paint!! Make sure it is safe for plastics! using the wrong paint will terminally soften the plastics. A spray on paint is best, if you want to avoid brush strokes.

Also, is it safe to spray a mouse without taking it apart?

no

btw... it would be massively help-full to know what type of mac your talking about.

There was a painded Q700 case in the hacks section here on AF; the guy explains what type of paint he used... he said it actually forms a bond with the plastic, as opposed to just layering on top of it:

http://www.applefritter.com/node/3437