Put simply, this problem has plagued COD since BO. Both BO and MW3 have had an extreme amount of criticism and reporting of poor gameplay experience due to lag, with matchmaking amongst other things being cited.
The matchmaking system, hosting system, servers, game add-on features such as theatre or customisation have also been cited as possible causes.
Both Infinity Ward and Treyarch have suggested customers do not have optimised internet set-ups which may explain some cases but not the sudden change or trend of increased lag experienced. Even so, more gamers play online at once these days, yet conversely fewer online typically leads to more lag reporting.
DLC and community fragmentation by alternate playlists OR alternate "hidden" playlists are also possible problems.
After the release of BO, each patch seemed to spring cries of "worse lag than ever", and the same has happened after patch 1.03 for BOII.
I don't have a single answer for you, but I know that just because you're playing against Americans doesn't mean a thing, really. The bars, lag compensation system and host lag buffering all throw confusion into the mix. I can have a great connection against people in my city or many cities away, I can have a terrible set-up with a neighbour as host, but an ok game with players from another continent. There are so many factors.
In short: something changed after MW2.
Hopefully it smoothes out some in the near future, MW3 ran decently for the last few months, and even before it was only noticably bad one out of every three games or so, at leat for me. I hope they put up a hot fix issue and patch notes page on here like they had for MW3, at least then I would know if something changed, what it was, and if the "lag change" was just in my head. Maybe I just had a horrible night last night, maybe I was playing against guys that really nailed down there playing style. Just a little information would be nice to figure out what happened. There were a few games that the other team held down their area's well, with well placed people sniping and runners in 2's and 3's. But there were others that were disorganized chaos and just spraying me down after I shot them, so it probably a little of both, but I've never had a run that bad that lasted so long before.