To sign up for the site, you need only a username, password, and verification method (email/sms/facebook/something to authenticate you as a person, but this wouldn't necessarily be tied to the account). The usernames are unique, and your personal url is domain.com/username or something along those lines (like reddit).
Then you can give your business card to everyone and their spammy recruiter, and if they follow up and visit your domain.com/username, they can hit the "request email" button, at which point you can choose which email to give them or ignore it all together. Potential problem is that unique usernames will mean that professional ones run out quickly. Not sure at what scale that becomes an issue....
In other news, if anyone here is going to be at hackMIT and would be interested in working on something like this, email me (its in my profile).