Brain Drain reversal? The Hare vs Tortoise hypothesis.
Since WWII, the US has benefitted tremendously from a steady "brain drain", whereby scientists (particularly physical science and mathematics) move to the US for permanent positions; this is partly because the US is large - in a given field, a particular European country might have 0-1 positions in any given year, and the US might have 5-10. The US was also richer and had more higher education participation per capita, so relatively more jobs. They also paid well (though I think the recent collapse of the dollar shows that even PPP adjusted exchange rates were anomalously tilted to the dollar for a couple of decades there). Finally it was often easier to move Europe-US than within Europe! Until the latest round of EU forced reforms, getting basic stuff like pensions and health-care cover was non-trivial, and there were (and still are) severe cultural barriers to hiring foreigners (eg the Netherlanders always did; the Belgians did not. Germany recently started aggressively hiring foreigners, but France and Italy do not; nor does Scandinavia, although that is changing.) So steady trickle of Europeans to the US, and spurts of other nationalities (lots of Russians in early 90s. Waves from China in mid-80s and mid-90s on; spike from India in the 90s (now apparently ebbing); lots of Eastern Europeans right now), but the trend is down, to the point where graduate admissions are getting tight. Oh, foreigners represent about 50% of the graduate student and above workforce in the US in the physical sciences.
If this is reversing, if there is not just a decrease in new people coming in, but an exodus of settled people, then the US science infrastructure could take a very serious blow (on the other hand, it would improve the job prospects of the few US nationals who actually hang in there and do physical sciences). Is this real? I don't know, not enough data yet, but anecdotally I'd say it is - people talk about leaving, they make semi-serious enquiries about leaving and more interestingly the Europeans (and Canadians!) are fishing for prospects...
So what is going on? It is not just money, or politics. It is partly opportunity, Europe is hitting the wave of retirements from the 60s expansion wave just as hard as the US, and possibly little later; and the anti-boom in Europe has been more severe than in the US, with some small cohorts (and in a rare field, a contraction of the recruitment pool can shrink prospects entering the pool to zero - no candidates graduating at home in a particular subfield in small countries! Particularly with enrollement in physical sciences down everywhere as a percentage of total enrollement).
However, there is an even more significant change: Europe is pulling ahead in science (and I don't just mean particle experiments and nuclear energy)!
There's a hare vs tortoise paradox here - the US has more funding, is more agile in allocation and is faster in responding to new results (cf the High Tc superconductors and what happened in the UK (for shame!) vs US; or planet finding - the ramp up in the US was much sharper and the US seized a lead after a Europe breakthrough (leveraged off Canadian and UK techniques...) - BUT, the Europeans hang in there, and they are less vulnerable to fashion flings and porking (yeah there is a "just return" and regionalism, but the Europeans tend to take science committee and panel recommendations terribly seriously, sometimes too seriously (see above on lack of flexibility)). However, the European system is less likely to lead to projects abandoned in mid-stream (yeah, sometimes they should be, but too often they should not), and there is less pressure for "what have you done lately", it is possible to assemble research teams that work steadily on consistent projects for a long time, until they own the field. Totally.
And this is happening: there is waste in Europe, there are incompetent research groups and idiot professors leading whole teams astray (and in the US too - cf some of the "earmark" expenditure in some of the more interesting places). But, Europe is pulling ahead (and I don't mean particular issue fields like stem cells, I mean basic areas like optical interferometry, quantum optics, materials, computing techniques. Inconceivable, I know.).
Search for Planets using optical Doppler radial velocity time series is actually a good example of this - the US took the early lead after losing the start (but did have the first detection of extrasolar planets at all in radio of course, with PSR1257+12 planets!), but the European teams have come back, and in my opinion have the lead now. The VLT is unbeatable right now, and they have better instruments. This will change (in fact IIRC the California group got an "earmark" which will jump them again to a lead, probably) but it will change back again, the European groups are not relinquishing this field. And the US can't just take supremacy in the field by leveraging resources or technology, it is a genuine race.
This, I think is the driver for any reverse brain drain - Europe is becoming as good, or better in some areas, to do science, and people go where they can. Living standard is also as good as the US, better in some ways depending on family status, and what will happen with US health care system (and if the dollar crashes seriously, as it might, all bets are off). Politics could also become a serious issue - if the US becomes more xenophobic, or the christian right makes serious political gains, some people will vote with their feet. Tenured faculty are immune from a lot of social fluctuations, but not all; nor do they all want to be.