Mid-April it was the white 16GB GSM iPhone 4 that was being swapped out at many Apple Stores. Now its black 16GB iPhone 4 and black 32GB iPhone 4.
Like with the 16GB white iPhone 4 constraint situation of mid-late April, Genius Bars without stock of the black 16GB iPhone 4 and black 32GB iPhone 4 replacements units have been instructed to provide customers with comparable iPhone 4S replacements. Not all Apple Stores are facing these constraints (but many are), and these constraints should clear up within a few weeks if the white iPhone 4 16GB constraint process follows suit here.
If you need to have your iPhone 4 serviced. Now might be a good time for a visit to the Genius Bar!