Affects Version/s: Minecraft 15w31b
Fix Version/s: Minecraft 15w41a
Windows 8.1, Java 1.8.0_25 64-bit
Apparently the End still isn't quite as "infinite" as the Overworld. If you teleport really far away using console commands, it will stop generating. This is far enough away that it's not likely an intentional limit to the size, as it's farther than anyone is likely to actually travel, especially considering you can't set your spawn point in the End.
1. Make a new creative world
2. Build and activate an End portal, or find one in a stronghold
3. Go to the End and start flying
4. Type "/tp @p 524200 70 524200"
5. Start flying southeast (F3 will tell you which direction your facing) and you'll soon notice the islands stop generating.
This has been tested with the following seeds: