I can see that the tax consequences of earning the attractive (albeit unrealistically inflated) locums rate are harsh -- and are yet another reason to vote Republican in '08 and keep the tax-happy Democrats out of power.
What would you say about the possibility of working for two practices at once? For example, could I work Monday through Friday at a practice, and then fill-in every other Sunday at another pratice for the inflated ($800-$900/day) rate?