I've been asked to support searching by email address in the social graph API plans search other social sites by email address to find your other accounts. This would be very helpful in helping you create profiles and find friends faster, but also could give Google a more of your personal data.
I suppose search engines could also do this by full name or username, but an email address is considered a unique identifier and lots of sites have you validate your email address after registering (though Pownce doesn't).
Pownce - I'll let you decide.
