Stephen Matheson recently wrote a very interesting article on the evolution of bats. It examines the genetic changes which led to the bats gradual transition from tree climbing mammals to night wings.
I do have one thing I'd like to add to the article: It states that "The fossils can't yet show us how paws gave rise to wings". This is true, there is not a sequence of fossils that show this transition (Bats are too fragile to fossilize well). However, there was a fossil reported a few months ago which definitely represented a transitional stage between tree climbers and bats.