So much...
Lets start wtih the U-boats. Leej, you might not believe that they were a war-winning threat, but Churchill (who was in a better position to know) thought so, and said so repeatedly. In point of fact, he believed (as did Pound and several other members of the Admiralty, who admittedly might be a bit biased) that the U-boats were the ONLY threat that could have brought Britain to its knees. I am not sure that additional production would have changed matters much, but if you want a POD where Britain has to surrender (or agree to a separate peace, in practice much the same thing), this is about the only realistic way to get it to happen...
The point raised several times about nukes being insufficient to wipe out a city wholesale (or kill Hitler if he was in it) is well taken, but ultimately unimportant. As for nukes not killing Hitler, I am not as sanguine about the value of his shelter(s) in the face of the devastation (blast+radiation+fire+fallout+rubble) that a nuke can generate. Even if Hitler does survive, much of the nazi hierarchy and its attendent bureaucracy doesn't survive, and that will bring about the collapse of the state just as easily as killing Der Fuhrer. The point that nukes in 1945 weren't fundementally different in scope from massed bomber raids misses the point. Nukes were 1 bomb, massed bomber raids were just that...massed. The Japanese suffered worse than Hiroshima during the Tokyo raids, but it was Hiroshima and Nagasaki that got them to throw in the towel.
A suggestion was made that the military would be more friendly to Hitler following the collapse of Russia, a point that is not supported by the historical record. Even after the fall of Poland, the swift success in the West, and the early successes in the USSR, military opposition to the 'jumped up corporal' was strong and intense. Once cities start disappearing in big mushroom clouds, the recriminations will begin, particularly if there is no effective counter. Perhaps not enough to spark a coup, but I rather doubt that the opposition will be willing to fight to the death amid radioactive rubble just because Hitler has a Samson complex...
The Germans weren't in a position to build their own bomb, period. Forget about the Heisenberg story (that he was deliberately sabotaging the German program, unlikely in the extreme given the way that the Nazis dealt with those they felt were traitors to the cause), the reactor that we found didn't even have control rods, and the German bomb designs lacked even the simplest concept of neutron reflectors. The German scientific hierarchy wasn't oriented towards open inquiry (they tended to simply follow the orders of the folks at the top), which means that throwing money/resources at the problem wasn't likely to generate much in the way of new approaches. Only the US had the resources to pursue multiple approaches to a hugely expensive project, and then build the delivery systems for it. One forgets that the B-29 program cost as much in RDTE as the Manhattan project...
As for numbers of bombs, wkwillis makes an excellent point that the US won't have that many bombs available soon, but how many does it need? Can you really imagine Germany fighting on after 10 nukes? after 20?... If there were a 'Black Christmas', with 10 German cities incinerated on 12/25/1945, do you really believe that a major crack in the German will to fight wouldn't occur?
Aktarian, your comment re: American will to fight is as incorrect as it is foolish. Why not take a look at the fighting in the Pacific, where nothing happened fast, and the fighting was bitter and often fruitless. As to why we assume that the Germans would declare war upon the US, the best reason is that this is exactly what they did. Why, given a victory over the USSR (assuming that they have won by 12/11/41) would they be MORE cautious? You are correct though, that they certainly would have followed up the fall of the USSR with strikes through the Med and possibly elsewhere, significantly complicating any US counterstrike. Since we aren't going to invade anyway, this is ultimately irrelevant...
Which brings us to the best post of all, and the one which has been neglected here. What if the US doesn't build a bomb? If we have a POD with no nukes...well, at that point I suspect the game is up, and the Germans will survive. Once you presume that they have all of Europe and much of the Middle east, it becomes very difficult to imagine that they won't have sufficient resources to sustain a long-term defense of their empire.
A final point, re: the Luftwaffe and the eastern front. The number of planes and pilots in the East was greater than that in the West for every year of the war, with the sole exception of 1940. Granted many of these were attack aircraft/bombers, but the resources are, for the most part, fungible. Without an eastern front, the Luftwaffe would be immensely strengthened, and possessed of resources that would make further development far more practical
