The subject of meditation isn't all that important, the focus is. Breathing just happens to be a great subject: it's simple and repetitive, it's always with you, it's something you can feel (you're supposed to pay attention to the sensation), etc.

I recently read Mindfulness in Plain English and really enjoyed it. If you're looking for an explanation and how-to (but not necessarily data to back it up), it's a great book.

Edit: Regarding cats specifically, breathing does not demand your attention (the way cats do) and it does not leave on a whim once it has received your attention (the way cats do). It's up to you, and only you, to focus and continue focusing.