Now that I've stored away my CD collection and use iTunes almost exclusively to listen to music, it happens very frequently that the phone rings and I scramble to find the iTunes window in order to pause the music, after which I have to do some additional window shuffling to get the window layout back the way it was in order to continue working.
Obviously there are many shareware or even freeware tools that allow iTunes to be controlled with the keyboard, but I haven't been able to settle on one that I really like. However, these days I take advantage of an iTunes' behavior that is documented in other hints: when the iTunes window is zoomed to button-only size, clicking on the controls doesn't bring focus to iTunes.
So what I do is drag the zoomed iTunes window to the lower left or right corner of the screen and partially off-screen such that the the play/pause button occupies the actual corner. For some reason, windows never occupy the last few rows of pixels at the bottom of the screen, so unless the Dock has too many icons so they get in the way, moving the pointer all the way to the corner of the screen will always position it over the play/pause button even when there are other windows on top that are zoomed to maximum size. A click will then stop or start the music without changing window focus.
Since the corner of the screen traps the pointer, it's not even necessary to really look where the pointer is going: just moving it a good deal to the left/right and bottom will get it in the corner. Switching to iTunes and choosing "zoom" from the menu will restore the iTunes window to its regular size and position.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040624101024150