Alan Bryant , Jan 21, 2010; 08:11 p.m.
You'd lose AF, but that doesn't work well with extension tubes anyway. More problematic is losing control of the aperture. You can still set it, but only with some difficulty (mount the lens without tube, set the aperture you want, press the stop-down button and hold it while you remove the lens - the lens will keep the aperture you set.)
You'd also lose IS and any other power-dependent features the lens has. But still, as extension tubes, I expect they work.
For what it's worth, there are other lenses - for example, the LensBaby - that have no lens-to-body communication.