Timothy R. Hurley (Salisbury University) has published Curing the Structural Defect in State Tax Systems: Expanding the Tax Base to Include Services, 61 Mercer L. Rev. 491 (2010). Here is the Conclusion:

The majority of states enacted a sales tax between the 1930s and 1950s. During that period, goods dominated the economy. States intended the sales tax to tax people on their consumption of these goods. To maximize revenue, therefore, states established tangible personal property as the tax base for the sales tax. Over the last thirty years, however, the economy has transitioned to services. State tax structures have not kept pace with this change in the economy. Consequently, in many states there is gap between needed revenue and actual revenue. To cure the structural deficit in states' taxing structures, states should increase the tax base to include services. “Given that the sales tax is intended to tax people on their consumption of resources, there is no reason why the individual who purchases services should not also pay tax on that consumption.”