I wonder if there would be any effect on the War of 1812.
I was thinking the very same thing actually, about what the lack of a 100 Days (by whatever means) would do to teh War of 1812, and I basically see this happening:
With the conclusion of the Napoleonic Wars basically, Britain can focus all its attention on the United States, and the War of 1812 goes either two ways from here (really three, but two for the sake of argument:
The first is that the British pour more soldiers into the United States over the next year or two, and the war there becomes a long and protracted struggle. The British get tired and leave sometime between 1818 and 1821 I'll guess.
The second option is that the Americans can't handle the influx of soldiers and as such the British are able to bring America to the table, and here they either demand New England be released from the US (if I remember right, just before the war New England wanted to secede over the embargos and trade restrictions). At this point New England either remains autonomous or basically gives up its sovereignty and is reincorporated into the British Empire as part of Canada.
The third option is the same as the second, but the Empire basically reasserts its control over the Americas, and keeps a heavy garrison in the country. The colonies are then made Dominions when/if the rest of the British Colonies, but especially Canada, are released. In doing so, they'd probably actually release the two together as a single entity, giving the side-effect of an apparant Ameriwank with the super-sized "United States."