This article in the NYT thinks that European and Japanese MNO's are coming to the States because there's no growth left in their own markets. That's (obviously) far from true.
I think the real reason is that American MNO's have done a nice job of opening up the mobile market in the past couple of years - they actually learned from the mistakes that their European counterparts made (and continue to make). Like obtuse and opaque pricing, idiotic marketing, feature creep, and, most importantly, price-gouging.
The reason there's no growth in mobile markets in Europe (and, to an extent Japan) is because MNO's have got the 3G proposition massively wrong - pricing, marketing, everything. I recommend you look at FOMA diary if you want to know more.
If there's any industry that's going to grow discontinuously in the next decade, it's mobile communications - it just may not be the now-incumbent mobile MNO's that see these profits, because they have absolutely no strategic insight.