I've been looking for something like this since I got my droid. Haven't seen anything in multiple searches through the market, several app sites or any of the android forums.
Things I think might be cool as options:
Ability to set up groups. On my days off, I'd love to be able to avoid any and all calls from work. If I can set up a group list (supervisors that feel the need to bug me on my day off
) where I can turn that group off and they all go straight to voicemail, I'd be a very happy guy. Just a bit of a pain to go through the 6-7 people and turn that feature on / off for every one of them. Then go back through and take it off again.
Icon in notification bar. - Shows that the forwarding app is on and functioning. Also maybe what group is turned to auto forwarding.
Also an automatic scheduler might be kind of cool if possible. Automatically turns certain groups on or off at a scheduled day / time.