There's a way around that, actually!
1) When you create the poll, insert </form> as the first thing in the Poll Name. (I'm not sure if this also works if you insert it as the first thing in the first question).
2) When you post the poll, insert <form action='http://www.livejournal.com/poll/
3) Voila! Now, when someone submits your poll, the "Success!" page will pop up in a new window, rather than taking them away from their Friends page/lastn view/whatever.