No, it is not, in most countries. As bitcoins are considered a good, it is generally considered barter. A contractor can accept whatever they want in return for their services, and there's specific taxation rules set out for it, at least in the UK.[0]
An employment arrangement, however, might require that a reasonable currency (either the legal tender of the country, or whatever the court decides is reasonable in countries which do not have a legal tender) is offered. Certain laws might allow the employee to opt in to being paid in Bitcoin anyway; BitPay supports this in the US via their Payroll API[1], and I have yet to hear of any legal challenges to it.
