Its a good spam oppertunity. Collating loads of email addresses could be very easily abused it doesnt really need an explanation.
The owner of the email address doesnt have a choice of whether they want to recieve the email address or not.
EDIT:
http://www.johnvey.com/features/gmailapi/ - Gmail
http://developer.yahoo.com/mail/ - Yahoo
http://dev.live.com/contacts/ - MSN
I cant think of any other providers but it looks hopefull, mabey a clever developer could make something for vBulletin