Why do you think so many of us are complaining about being in limbo - personally I think it's unethical to sell pv at the minute, but you wouldn't be alone.....
Firstly, I'm a customer not a supplier, so you'll have to take my opinion in that light.
I would suggest being honest with the customer. Point out that to the best of your knowledge the FIT will be 21p. Point them at the newsletter from Greg Barker which seems to confirm this (http://www.electriciansforums.net/p...eholders-12th-december-2011-a.html#post425099). However, admit that until the result of the consultation is published you cannot offer any guarantees.
If they ask about the court case, point out that this may mean that some installations post 12 December may qualify for a higher rate but this is not certain, and even if the cut off date was changed there is no guarantee that their installation would qualify.
Include (in large print) an acknowledgement from the customer that you've given advice to the best of your ability but that no guarantees can be made about the tariff due to the current circumstances and that the customer understands the risks.
You may lose a few sales (or get people deferring), but I think most customers would respect this.
Oh, and get it in writing beforehand that REA/MCS are happy with the above process, so that you are covered!