Google is aware that people are building tools on top of their UI. If they were against it, they could pretty easily thwart it (e.g. by randomizing their HTML/CSS, or by serve different versions to different users). The fact that they haven't taken time to do this is, I believe, evidence that they aren't hostile to these efforts.
Also, I've personally been in contact with people on the Gmail team about this kind of stuff. They are, generally, supportive, but they're very clear that they'll make no guarantees about stability.