I know its been discussed a fair bit. Stores like M&S, Topshop have gender neutral fitting rooms, and I know my local H&M and Primark are not fussed about men and women/girls being in the same area as I and DC have observed (with not exactly floor length door cubicles). Not necessarily those dressed in womens clothes already, maybe men who like some of the womens section and thats fine in itself. We still try things on in those stores, but I guess its in the back of our mind to some degree.
How you police such a policy is of course another matter, especially those of ambiguous appearance or in the early stages of gender transition.