Via the Second Life forum I made my way to a Gamesblog interview of SL founder and CEO Philip Rosedale. From the interview:

In May 2005, the total amount traded in-world was USD$1.47 million. There were 1.3 million transactions between 19,500 unique users.

And to think this is really still in its infancy.

(edit: C|Net has latched onto this interview. you can read that entry here)