I couldn't find anything about this using the search, so I thought I'd pass it along.
Anyone running Tiger and using a mouse with a scroll wheel (I assume a Mighty Mouse would work) can magnify the screen image by holding down the Control key and scrolling. It's the same as holding down the Option and Command keys and tapping the + key. Scrolling up magnifies.
One site suggests Apple implemented this with a recent Tiger update in preparation for Leopard, with the leap in logic being that Leopard will be resolution-independent, as are pdfs.
It's great for small pix and avatars, too.