Well, after digging, it turns out the Messaging app is closed source. I want to add Google Voice support (because the GV app sucks), but I doubt I'll have the patience to write an entire, good texting app from scratch.
Does anyone know of an open source SMS app for Andorid? SMS Popup is open source but it's really meant to work with your default SMS app. Still haven't decided if it makes sense to start from there or from scratch if I can't find anything else.