A clever spammer could create a website that relays any sort of captcha challenge to users who are willing to solve them for .01BTC each. It has been done before, but using porn for a reward instead of bitcoins. Same principal.
Not that clever since he'd be paying .01 instead of the direct access payment of .001
A more substantial deposit that is returned after the account doesn't spam is probably better, that way it is only a cost to spammers. It only really works in communities where most people will have coins and even then could be annoying, but so is spam.