Trust- / investment-based social deterrence
Possibility No. 1:
Friends will lose money when you derail, just like yourself (but not necessarily the same amount). This augments the deterrence and now your reputation is at stake.
Possibility No. 2:
Friends / investors pay money up-front. They only get the money back if you reach your goal / do not derail.
Many other forms of this idea can be conceived of. I think trust- or investment-based incentives may well be the future of Beeminder.

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Admindreeves (Cofounder, Beeminder) commented
Thanks Samuel! I'm not convinced yet but I love to debate these possible future directions. See also http://healthrally.com
If anyone tries experiments like this on their own (like convincing a friend to mirror your beeminder goal so they cough up whatever you cough up) we definitely want to hear about it! The akratics anonymous list is a great place to discuss such experiments: http://blog.beeminder.com/akratics