Principia started this (I do believe), though I don't remember if it's a 1" top, 1 1/8" bottom, but the bearing diameters are different top and bottom. I will think of the benefit in a little while, though there is something to do with the load placed on bearings, and the ability to use a larger headtube to make a better joint for the downtube. Also, Isaac bikes (engineered by Principia) do this (I think, I will have to check).
If you had asked me who makes the finest production aluminium bikes, I would say that either Principia, Klein, or Cervelo. I am not counting the customs like the Mandarics or anything like that. But if Principia did it, I would definitely take notice. They still don't paint frames (I don't believe; only anodise) and somehow anodise the inside of the tubes (unless I misread their website).*
What I am saying is that there has to be some sort of benefit if somebody who does aluminium bikes right did it first. But it could also be gimmic.
*Edit: they do paint frames these days, but to find out their sales pitch on 1 1/4" bottom, 1 1/8" top head tubes, go to:
http://www.principia.dk/