Currently there is no federal sale tax being imposed on the sale of electronic cigarette or as some refer to them as e cigs in the United States. This does not mean that it will not happen as the federal government can insert its power under the commerce clause at any time.
Sales taxes are being imposed on the city and state level in various different areas. Legislative bodies throughout the United States have also sought to pass legislation proposing to impose sale taxes on the sale of e cigarettes but most have not passed yet. As states run into more and more financial difficulties, it makes sense for more and more of time to seek additional areas of revenue. So the e cig industry is a popular target.
The FDA has announce their intent to decide if they will regulate this industry. Once that decision is made, it will not be long before Congress seek to impose taxes especially if e cigarettes are deem covered under the definition of tobacco related products.