As I understand the law, if you do a favor for someone with no expectation of return, and then they do a favor for you with no expectation of return, then no taxes are owed.

Of course, the IRS can't read your mind, and neither can the tax court. If your usual business is selling radio ads and his usual business is driving a snowplow, I doubt if the IRS would question it. But, if your usual business is setting up websites and his usual business is cutting down trees, the IRS might look askance.

In practice, as long as the exchanges are informal, I don't think the IRS is going to care. In the past, though, they have gotten involved when people set up bartering networks that let people keep track of how much they're owed. (There are bartering networks where you get 10 points for feeding my cat while I'm out of town, then spend them getting a babysitter in for your kids, after which the babysitter can spend them at the local video store. The IRS has gone after people involved in such things saying that each point was worth $1 or some such.)

Legally, though, if you exchange services with someone, you both owe taxes of the value of the service performed. If each service is done as a gift, with no expecation of a favor in return, then it's tax-free.