Looks like because of the agency pricing model that some publishers are using there is sales tax on certain ebooks depending what state you are in. I first saw a mention of this in a blog post by Mike Shatzkin called Most dramatic publishing event of 2010? Introducing agency pricing!
Running some searches I found some additional blog posts that discussed the issue.
Sales tax on ebooks and rising prices
Amazon Says Agency Model eBooks Are Subject to State Sales Tax
This article is particularly interesting because they tell what publishers are using the agency model and it what states they charge sales tax.
How does Diesel eBooks handle Sales tax?