There are a number of who think so.

The designation of lawful and unlawful combatants is set out in the Geneva Convention.Lawful combatants are nonmilitary personnel who operate under their military’s chain of command. Others may carry weapons in a war zone but may not use offensive force. Under the international agreements, they may only defend themselves.

The amount of force being used in Iraq by security firms like Blackwater has raised questions.

I would also add that under the UN convention on mercenaries (see my posts here and here), some of the activities, like protecting military supply convoys in a war zone are clearly illegal.

While the US hass not ratified the convention (Neither has the UK) the position of one of the many countries that has might very well be to treat these folks as common criminals, or worse.

Some readings Islamic law call for the prompt execution of mercenaries.