Double-diffusers, we're assured, are banned from Formula 1 in 2011. Now, unlike Marco Piccinini, I don't go to sleep with the FIA yellow book under my pillow, and I haven't seen how the regulations are specifically worded to enforce this ban. Nevertheless, the easiest way of doing so might be to target the means by which the double-diffusers are currently fed.
Recall first that the underneath of a Formula 1 car consists of a reference plane astride the centreline of the car, and a step plane 50mm above it on either side. The upper decks of the double-diffusers were fed, either by holes in the vertical wall joining the step plane to the reference plane, or by holes in the horizontal step plane which lay in the shadow of rear suspension arms. Thus, to outlaw double-diffusers, one merely has to stipulate that: (i) the entire undertray, consisting of reference plane, step, and step plane, must be a continuous surface; and (ii) holes in the step plane are not permitted, whether they're in the shadow of suspension arms or not.
So, is it possible to circumvent this somehow, and achieve similar results by a different means? Well, the first thought that springs to mind is this: what's the difference between a joint and a hole? How about making the underbody from separate pieces, with a joint between the step plane on each side and the reference plane. This would still form, with the car at rest, a continuous surface even if it were made of multiple pieces. One might then allow the joints to open up in a certain place under aerodynamic load, feeding the upper deck of a double-diffuser. McLaren used a multi-piece underbody in the first half of the 2009 season to enable better access to the KERS battery pack in their right-hand sidepod, and with KERS returning next year, this might provide a perfect pretext.
The visible nature of the double-diffuser rear exit might, of course, be thought a giveaway to the fact that one was flouting the regulations. Perhaps, however, one could implement a diversionary tactic, feeding air from channels in the flanks of the sidepods to the upper deck of the diffuser. Other teams copying this might find that it offers them no benefit at all...
Perhaps someone better informed can enlighten me on the exact wording of the new regulations?