For what it's worth, I don't buy into the weekend theory either, at least not based on the explanation that Republicans are more likely to be out of the house than Democrats. I hear this claim all the time, but I am not sure that I have seen any hard empirical evidence to support it. Indeed, Time and Gallup often poll partially over weekends, yet they seem to have no problem finding Republicans willing to talk to them.
The reason Republicans are difficult to reach on weekends is that a disproportionate number of them work on weekends, frequently at jobs they've created. They are also more likely to screen their calls or have unlisted numbers.