No one I've talked to seems to know where TV Guide got the idea that there would be a short after the show on March 21st; however, from the description "set in the 1940's" it would have to be a short that was originally planned to air at the end of Bart the Daredevil. It was called "Nazis on Tap", and started off with Harry Shearer doing an announcer's voice saying something like "And now, here's a short from the Simpsons archives, from the year 1944." Homer was in a 40's version of Moe's Tavern (for some reason, the bartender was a dog; perhaps someone else can supply a more specific reference for why they would have done this. I just have a vague feeling that this sort of species mixing was common in a certain style of cartoon), relaxing after a hard day's work in the defense plant. Hitler sits next to him at the bar, and starts pumping him for information about weapons and invasion plans -- Homer of course answers all the questions. Then Bart comes along, spots Hitler, and chases him. The final scene is of Roosevelt giving Bart a medal for capturing Hitler; when Bart starts to chase after another man who is present at the ceremony, FDR says "No, no, that's Joseph Stalin, our friend!" The short ends with Bart saying "Catching Hitler was neato! Next stop, Hirohito!"
Anyway, as far as I know, all that was done on this was the script and voice track (hearing which provided me with virtually all I know about the short), and the character design (the dog, the world leaders, and the 40's versions of Homer and Bart). At that point, someone decided that maybe they didn't want to go through with the idea (as I heard it, it all started as something of a dare to find something funny about Hitler), and the last my source knew, that was that. When it appeared in TV Guide, therefore, it was something of a shock. It was of course possible that Gracie had moved the project to another animation studio (you may have noticed that the Deep, Deep Trouble video wasn't done by Klasky-Csupo; there's been considerable friction over this, and supposedly the contract for next year states that Gracie Films will do all their Simpsons projects with K-C) and gone ahead with it. Since it hasn't actually appeared on TV, however, I can't do anything more than speculate.
Don't forget when Itchy and Scratchy teamed up to fight Hitler.
Then I can write a washing bill in Babylonic cuneiform, And tell you every detail of Caractacus's uniform; In short, in matters vegetable, animal, and mineral, I am the very model of a modern Major-General.