Oh, I'm allowed to, absolutely. But the simple, brutal truth is, I don't want to. I find designing for 4E to be much more enjoyable, and much more to my taste, under something like 85% of circumstances, if not more. (It's the only reason I haven't been pestering Paizo for work.)
If I ever have a large gap in my schedule where I can't find any 4E work (or fiction work), I'm not saying I'll never go back to 3.5 design no matter what--but I don't think it likely, and it's not something I especially want to do.