Who bells the cat? Will you write the code?
Now that Trike has time to implement code written by engineers like Lucas, we could certainly implement this if we think it's important. I'm pretty ambivalent about it, though, because the proposed change would allow people to contaminate themselves before voting.
As far as I can see, the only way to see the results of a poll is to vote in it. If for whatever reason I don't want to participate in a poll whose results I am interested in, I vote randomly. Of the ~10 other LWers I know, at least 2 do the same. For the sake of accurate results, please let people see poll results without having to vote (and ideally without having to log in).
Response to obvious argument against: You can restrict people from (voting after they have seen the results).