What's a good way to add mobile-specific CSS to an existing web page?

The requirements are:

* Non-mobile browser view should stay as-is
* Mobile browsers should get a different CSS (that removes certain elements and simplifies the page)
* Mobile browsers of interest are: iPhone and Android. Win7 and WebOS would be nice to haves
* The smaller the number of changes the better

I'm looking at HTML5Mobile boilerplate and various other solutions for the long term, but for the short term I'm looking for a low effort CSS based solution.

I'm hoping for a client-side only solution (as opposed to serving a different page).

Recommendations? Is there a library / article / code snippet that'll help me out?