I think there have been at least three since I started playing. One was a complete character wipe of the database. Another was a money and inventory wipe caused by a quest glitch. The third was a money wipe necessitated by a player sabotaging the game. This doesn't include the wipe when CB was released, which happened before I started playing.
There was a mini-wipe when the inventory was changed such that you could only hold X items in any particular slot. I had hundreds of rubies in my inventory, and they were lost when this change was made. I suspect that other players lost items as well. It's no biggie though.
