In the process of building such a service, we've already spoken to a couple lawyers about these issues and it's quite a legal gray area right now. We're working on ways to structure the system to avoid these particular requirements, at least in the short term. It really comes down to how you implement the bartering platform itself as well as the terms under which exchanges are made.
superfluid has functionality for commercial (with IRS-reporting, per jambo) and non-commercial efforts. So it handles both the barter exchange and favor-bank sort of model. -The IRS has exclusions for non-commercial services.
superfluid:p2p is international, superfluid:business is US-only, for now.
Sadly:
The IRS requires barter value to be reported as income: http://www.irs.gov/taxtopics/tc420.html
and barter exchanges have to send out 1099s: http://www.irs.gov/businesses/small/article/0,,id=113437,00....