How odd, I've also just returned from a US trip and was pondering the exact
some thing while I was there. I asked a friend out there (also a DIYer) and
he said the flush worked in the same way (at least in the pan, the cistern
part tends to use a "flap valve", which is now available in the UK). As we
had along wait at the airport, I had some time to think on the problem.

The "throat" of the bowl seemed much smaller and there is a large "pool" of
water in bowl. (Good idea to keep the bowl clean). I wonder if these factors
combined give a "quick flush" that looks like a vacuum. The U seemed quite
tight, so maybe the action is like this:

Bowl full, level will form at the U top lip. When you flush, a water is
added slowly (compared to the UK) until there is enough to start the flush,
which progresses quickly as there is a lot of "head" in the bowl. So fast,
the bowl empties before enough fresh water gets in to maintain syphon level.
Syphon stops and bowl refills.

Yep, I think the crucial thing is that it is actually a syphon, so it
continues with little head of water to maintain it. With our wash-down
pans, water added just pushes out the "stuff" like a normal u-bend,
which is why the new 6 litre things are so feeble. I've just seen a
description on
http://www.howstuffworks.com
There is a jet which aims water at the syphon to get things going, and
little water goes down the sides, which is I think the other reason for
the large disconcerting pond of water that is present - there is little
wash-down action that occurs, so everything must drop under water.

Any other interesting observations about the states ?
One thing for sure, the prices are never what they seem, what with tax,
tips to be added on. Canada seemed even worse for that.

Cheers,
Simon.