This is not uncommon at all. In fact, from 1999-2001, I worked for dotcom startup in San Francisco that was doing exactly this. Alternative economies are very much NOT unconstitutional, as long as they provide the government with one very important thing - taxes. The IRS requires you to declare your trade income, and as long as you do, it's all legit.
US currency is essentially a barter system, and it functions very similarly to the barter system we're talking about here. Dollar bills are essentially worthless - they're just paper. We all collectively agree that $1 is worth something, and attach a value to it. Essentially this is what they're doing with Ithaca Hours.
The panel of the comic where they talk about interest is a huge key to why people barter today. Say you own a restaurant and your water line breaks and you need new carpet, NOW. You have zero cash, and you don't want to pay interest on a quickie loan to get the carpet. So you go to someone in your barter system, they give you carpet, and in exchange you give out meals to people in the system. There's no interest, and you still get your carpet.
Barter will never replace cash, but there's a reason barter still exists today. It has its uses, as a secondary economy to the standard economy we all know.
Keep wreves in General Talk in 2011!