It's kind of surprising that Heroku hasn't cracked this nut on their own. Perhaps it isn't in their best interest to scale down? Scalability certainly is often touted as one of the benefits of "the cloud".
Unfortunately auto-scaling is incredibly application specific. It would be a great feature, but it seems likely to be quite difficult to make a particular solution that is useful to more than a small subset of users. You can get some sense of this from the other comments above (ie how analytics wouldn't be a good scaling metric for them).