Maintainability is the big reason not to inline CSS. Inline CSS is the most specific CSS you can write and cannot be overruled by other styling from within a CSS file, unless specified by !important (IIRC that is).
That and the fact that inline CSS isn't cached, like a regular CSS file is :)
I wonder why people suddenly think that inlining CSS is a good idea. I find it really confusing to be honest.
p.s. Don't forget about progressive enhancement... ;)