Voting to "punish" a party?
I think its possible that a lot of people may in fact do it, I know I've expressed my distaste with the Republicans while being a Republican myself. However, I'm voting for the opposite party because I believe what they're shooting for is the most part, right. While I will not be seeking to overturn my Congressman or Senator (Republican, Democrat respectively), there are names on a list of Republicans that I'd like to be seen taken around back and removed from office. The Republicans have messed up a lot of stuff, and I know just as well that the Democrats happily went along with a lot of their policies too, but either way, I'm looking forward to having some fresh leadership (from both parties) in both Congress and the White House.
Electability versus Political Stance?
The easy answer is to say that you should vote for who is best aligned to you, the reality is, the American political system does not allow for it to make everyone a winner. Unfortunately we are not as "lucky" as Germany (for example) where proportional representation allows for a greater number of parties who will cater to the variety of issues that people care about most. Because of the way our government has been set up, and otherwise evolved since the early days of the republic, we are otherwise left to a two (and sometimes three) party system. Politics is about compromise, simply put, and coalition building is what the Republican and Democratic parties have to do to maintain their power on the national stage.
This is where visible political movements become key in deciding the platforms of the major parties, and its why I'm both happy and disappointed in the performance of the third parties this year. I'm happy that Bob Barr and the Libertarians have received what otherwise positive press they have, and for that matter, breaking the "alliance" they normally have for Republicans... But I'm disappointed that the third party has been split into thirds with the addition of McCinny (sp?) and Nader. This year would have likely been a good shot for an Independent/Libertarian to grab 10-15% of the vote at minimum.
Just as an example, I voted for Ron Paul in the Michigan GOP primary despite him "not having a chance" against Romney or McCain. I thought it was important to at least make my vote in favor of him to legitimize his position in the GOP, win or lose. The difference is, thats a primary, not a general election, where far greater things are at stake. There are many people who believe that it is a "waste" to vote third party, and I can see where they are coming from, but at the same time it is a legitimization of their platform, and consequently it will alter what is offered by Democrats and Republicans later on. But, I'm not taking my own advice this election season...
Predicting the election to decide votes?
I think its natural that this would occur as the technology to conduct, process and share survey information increases. It is disturbing in some aspects to see the way in which it is flashed in front of us daily, almost like a score in a videogame to tell who needs to do better at any given point... But these are the tools that campaigns use to tone their campaigns, to get votes, and to get elected. Honestly, I like seeing the numbers on occasion, and I do like to check up on them somewhat regularly just to see how my candidates are doing. However, I've done enough work with them before to know that they are only estimations, and most-likely, they will be proven at least marginally wrong on election day.
As to how that effects my vote, well, I'm uncertain. I think its safe to say that if I were "a good Republican" I'd still vote for McCain while down 18% points in much the same way I did for Dick DeVos when he ran for Governor two years ago. But, I understand the point of your final... point. Big numbers and low numbers can both encourage and discourage votes, and that is why we keep hearing both Obama and McCain saying "no matter what, VOTE!" out on the stump. I think now, the only crazy thing that I haven't gotten used to exactly is the outrageous amounts of math that have gone into predicting the outcomes of the election by some organizations, which I find fascinating within themselves.
FiveThirtyEight.com is a great example, using a system based on sports statistics to predict the election.