While I'm sure this is possible, I'm not sure how complex it would be. The polling system for the forum, if I recall correctly from trying to modify it in the past, is quite difficult to modify. I've kind of put a temporary hold on any new modifications or changes to the code here until the next upgrade comes out. I anticipate its release sometime around the end of the year. It's going to be a very major upgrade.
I skipped the last upgrade because the amount of work required nullified any potential benefit we may have seen (and because I would have had to rebuild every single post over the course of several days on our 1GB posts table).
The next upgrade, though, I will definitely be doing, as it is basically a complete rewrite of the whole forum script. There will be tons of new features, tons of improvements, and an entirely new skin (although based around the skin we currently use). There is a possibility that nulling votes on individual questions within a poll will be a default feature, but I will have no way of knowing until I get my hands on a test version.
I think the short term solution is, like Wertz said, to make sure there is an option for "none of the above" or something like that. Of course, that puts the burden on the poll starter, not really the ideal way to implement this. But for the time being, it will have to do.
I don't see why there would be any problems with modifying a pre-existing poll to add a "none of the above" option, so if you stumble across a poll without the option and you would like it added, just contact Jaime or me and we'll get it added.