This, from the contest rules:

> CHALLENGE ENTRY BRACKETS WILL ONLY BE ACCEPTED IN THE CHALLENGE FROM THE FIRST FIFTEEN MILLION (15,000,000) ENTRANTS WHO COMPLETE STEPS 3(A)-(C) BELOW (THE "REGISTRATION PROCESS"), AS DETERMINED BY YAHOO! INC. ("YAHOO").

Edit; also:

> LIMIT OF A MAXIMUM OF ONE CHALLENGE ENTRY BRACKET (ONE ENTRY) PER PERSON IN THIS CHALLENGE.

I mean, aside from the fact that this poor site would explode if someone tried to register all the possibilities.