I would love to have beeminder take some amount of money as deposit and keep it or donating it in case of me failing my target! I would never give out my credit card information as that is just way too much of a headache to settle a dispute. Not to mention, with bitcoin as payment option you can accept customers world wide and circumvent the horrid credit card fees.

What about giving Beeminder a private key for an account that has enough funds and Beeminder does a transfer if you derail?

I should note, though, that there is apparently exactly zero demand for this, based on the balance of the bitcoin address on http://beeminder.com/donate

It still may be a good marketing move, as Karlson points out. But I'm not inclined to prioritize it unless there's some clamor from existing users.

How would this even work? Would you need some sort of escrow account to make Beeminder bitcoin-compatible?

I would really like to use Bitcoins instead of my credit card because of the high fees.
https://bitpay.com/pricing

Additionally, accepting Bitcoins in itself is already a good marketing move (I’m looking at you
reddit.com/r/bitcoin) potentially alerting a large userbase to Beeminder.