Here's a great Ask Metafilter thread on maintaining economic balance in massively multiplayer online worlds. A lot of basic questions get asked, answered, and reformulated here — it's a great introduction to the subject of virtual economies.
EVE Online has had a staff economist for about a year now, I think, as mentioned above. I play EVE, and a large part of my time in-game is spent making economic decisions: what to manufacture, where to buy raw goods cheap, transportation costs and risks (20 jumps through low-sec space to buy low might be worth it, or I might lose my ship and all its fittings)…and, yeah, it's really tedious but still somehow fun.
The great thing about EVE's economy is that, with the exception of skill books and basic blueprints, everything in the game is player-created — from the lowliest ammo to the most gargantuan battleships. So prices can fluctuate wildly across regions, and wily traders (with better ships than I can fly) can make mucho moolah off of those shifts.