Looks cool - although my cynical assumption is that this product a) would work great in demos with a simple number of folders,etc and b) will be bitch to maintain given when Google changes the UI or generated code / structure.
Having looked at the gmail generated source there isn't a human generated component so something trivial like $('#button_bar').appendChild(..) doesn't work requiring more clever approaches (and perhaps the streak guys are insanely clever). If I had the time I'd pick apart the extension source to see how streak works.