I don't know about anybody else, but I find it eerily creepy how accurate LinkedIn knows my connections. Any guesses how they are able to show seemingly accurate results?
Those people it's suggesting have probably e-mailed you in the past, or are in the networks of people that have e-mailed you in the past. LinkedIn matched your e-mail to their address books, which they gave to LI when they let it connect to their Google/Yahoo/etc mail accounts in the past. If you sign up with a fresh e-mail address nobody's ever seen, you'd have no initial recommended connections.
LinkedIn also keeps track of what profiles you've viewed, and uses that to guess people you've known. I believe it may also build a potential connections list based on the IP address as well.