For future assistance to anyone else, there is a "feature" in EPO 4.5 which can cause this to occur:
Although the technote doesn't explain my situation above, I added the "workstation" tag to the "Run on" settng, and kept "Server" and "Windows 7" as the exclude tags. Suddenly my Windows 7 machines no longer want to run the above task!
So if you have an Exclude tag, then you also need an include tag. Presume EPO 4.6 correctly handles this.