Bloomberg BNA recently conducted its annual Survey of State Tax Departments, and the results show the many ways that states apply their definition of nexus - the amount and type of activity a taxpayer has to engage in to be subject to a jurisdictions taxes - to include activities you and your clients may not be aware of.
States are struggling to preserve an eroding tax base in a changing economy. Its a trend that your clients should guard against.
(They should also bear in mind that different nexus standards apply to income tax and sales tax. For corporate income tax nearly every state determines nexus based on economic presence, which can include a wide range of business activities. For sales tax, states are required to determine nexus based on physical presence.)