After signing the message, the contents of the key could be spent too, not just compromised.
Yup.
I'm not sure the use case for this. It doesn't solve the problem that Certified checks do:
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http://en.wikipedia.org/wiki/Certified_checkThere is one approach that might work.
I wonder if this would be useful. Consider a third party that prints a bitcoin check. So I order one and send it to you. As the buyer, I know the Bitcoin address of the check.
You get this paper bitcoin check (which shows me as the "Pay to the order of" on it). That's just cosmetic but will make it so that this paper ins't really traded person-to-person like a physical bitcoin might be. So the check would have the private key under a hologram and that would let me verify that you had added funds to it but have not yet spent it.
There's nothing stopping you from spending it, but at least I can be pretty sure the funds weren't traded away, and thus that you still have them at any one point in time.
At the same time, you retain full control of the bitcoins at all times. The only risk is if the check printer couldn't be trusted to not have kept a copy of the private key, but that's a different risk that can be marginalized.
