New York, Pennsylvania, Nebraska, Delaware and New Jersey have the aggressive telecommuting rules. If you work outside one of those states as a job requirement, you are only subject to that state's income tax on the days you work there.
But if you work outside the state for your own convenience, you are subject to that state's income tax on all your income. For guidance see: http://www.journalofaccountancy.com/issues/2009/jun/20091371.html
