AMD is most likely going to have some form of refresh on 40nm to counter Fermi (depending on performance) and then make a test chip on 28nm before moving on.

The R800-series is the last of the R600-derived architecture supposedly, and so whatever comes next will be a new architecture, which gives AMD even more incentive to test out the 28nm process first on an existing architecture before moving on. RV740 probably saved AMD a ton of money on the transition to 40nm