I think the shy-Conservative factor has been pretty much eliminated in polling since 1992. While the polls may be underestimating the Tories by a point or two, I doubt's that much more. I think shy-UKIPers are more likely, especially since there's been a difficulty with pollsters has to how to manage the UKIP vote.
On tomorrow's polls - we are expecting Ashcroft, Populus, and the latest YG. Be weary of a potential Bank Holiday effect - this could also be the same for tonight's YG, and we may not get a more accurate picture until Wednesday. For Ashcroft, I'd be shocked if he doesn't produce some kind of Conserative lead (his polls have tended to show more larger Tory leads in particular). It'll be Populus, IMHO which will be the interesting one, out of all of tomorrow's polls as I doubt YG will move too much.