Still think they're on shaky ground. From the IRS page:
"Bartering occurs when you exchange goods or services without exchanging money. An example of bartering is a plumber doing repair work for a dentist in exchange for dental services. The fair market value of goods and services received in exchange for goods or services you provide must be included in income in the year received.
Generally, you report this income on Form 1040, Schedule C (PDF), Profit or Loss from Business. If you failed to report this income, correct your return by filing a Form 1040X. Refer to Topic 308 for Amended Return information.
A barter exchange or barter club is any person or organization with members or clients that contract with each other (or with the barter exchange) to jointly trade or barter property or services. The term does not include arrangements that provide solely for the informal exchange of similar services on a noncommercial basis."
Note that the IRS doesn't care whether the valuation is on a cash or time basis. You are definitely under the IRS rules if the work you perform in the hour is your profession... that _might_ be where there is wiggle room for HourExchange, but the explanation HourExchange gives doesn't hold water (in my non-professional eyes).