Exhaust-blown diffusers will effectively be banned in Formula 1 from next year, but there may be other ways of blowing the diffuser, and generating the side-edge vortices which appear to be crucial to maximising diffuser downforce.
For example, from 2014, Formula 1's engine formula will change from a normally aspirated 2.4 litre V8 to a 1.6 litre turbo-charged V6. The turbine in such an engine is constantly generating compressed air. Moreover, the inlet manifold of a turbo engine has a blow-off valve, specifically designed to release pressure when the driver lifts off the throttle or the throttle is closed. The blow-off valve could be vented down to the sides of the diffuser, providing vital extra downforce when a driver comes off the throttle turning into a corner.
From 2012, the regulations will prohibit exhaust-blown diffusers by stipulating that the exhausts are moved to a location in which they cannot influence the diffuser. These new regulations, however, will say nothing (as far as I'm aware) about blowing the diffuser with compressed air from the inlet manifold of a 2014 turbo engine!
Unfortunately, there is at least one potential snag: the Wikipedia entry on blow-off valves claims that "Motor sports governed by the FIA have made it illegal to vent unmuffled blowoff valves to the atmosphere." There is no citation, however, so it's difficult to ascertain if this is true, or even if it will apply to the 2014 F1 engine regulations. In fact, this is presumably something yet to be determined. Worth keeping an eye, then, on how those regulations are finally worded...
In the meantime, the teams could use compressed air cylinders to blow the diffusers, perhaps just for a qualifying lap. The primary declared purpose of these cylinders would be to supply the pneumatic valve system in the engine, of course, but as a safety measure, it might be necessary to vent excessive pressure. For safety. And cooling.