My guess is what's happening is that people really want an alternative to Harper, still remember sponsorship and also want an alternative to the Liberals, and along comes Layton as the only leader appears trustworthy.
An NDP majority would be a horrendous nightmare. Arguably an NDP minority government is no worse than a Conservative minority government, in that the other parties can keep it in check from doing the sorts of ludicrous damage that either of those extremes would do if they could do.
If the NDP obviously pulls ahead of the Liberals (and it seems to have) then the strategic voting gets interesting--people who would have ordinarily voted Liberal may vote NDP in order to block another Conservative government. That would have been a ridiculous thought a few weeks ago, but logically right now if you wanted to vote strategically against the C's that's what you would do.
What makes it viable that Layton could actually win and form a government is the possibility that the NDP could sweep Quebec, and appears to be the only Federal party capable of doing that.