In order to comply with LGPL, Sparrow has had to allow people access to their source. Despite this, one cannot redistribute any work based on these files. Kind of a bittersweet result.
Update: Looks like I got this a bit wrong. It isn’t open source, per se, but it they’re full versions of both the iPhone and Mac apps which you can compile yourself. Probably. I compiled the Mac app fine, but the iPhone version is throwing up all sorts of errors.