Beginning this week, more retailers will be charging sales tax. This was a result of an agreement between 38 states (and D.C.) and some major retailers. Internet News claims that this won't affect sales, but I beg to differ.
One of the major reasons I choose to shop online and not at brick-n-mortar is because of the lower prices. A major factor there is the lack of sales tax (granted I'm paying shipping instead, but now with the combination of the two it may be too much). If I will pay the same amount, or less, at a neighborhood store, why wouldn't I? Service and returns are much easier and browsing is much more enjoyable.
The one major exception will be the smaller online retailers, because they usually provide better products and the price isn't the motivating factor. I think we'll see more of a reaction in the coming weeks and I'm guessing it will be more negative than positive.