It would be easy for someone or a forum to rig the result (just look at that whole Erector mess last year, or the 2007 competition to get a line in the Movie being hijacked by a non-TFs community). I don't see a reason why anyone at Hasbro would rig the results, because the result is meaningless to their brand's direction or publicity.
They choose most of the characters anyway, so I doubt that they would interfere with the one "olive branch" they have given us fans (in terms of having a say in some way).

Judging by the responses here and at the HOF Dinner, I go back to my first comment, about someone interfering. Arcee and Megatron sounded like the more popular choices, but the vocal responses at the dinner, and the amount of votes they got over Wheeljack in the nominations round.