This move was inevitable. It has happened the day before the Overture bids are mandated to go to the 10p minimum. Espotting got a bunch of free PR on the back of Overture's decision.
I think for as long as Google are pre IPO they will dine out on this. They can probably give enough revenue away to cover the difference in any JV anomalies and because they get more searches their 4p clicks generally will get more revenue than the 10p's on Overture and espotting.
Mirago have always been at 10p minimum and don't seem to have suffered, but it will force several smaller advertisers to look seriously about the viability of PPC.