Speaking as a foreigner myself, who has done a post doc at UC Riverside and now works for a pharma company in San Diego, I feel Andre is right. The US Universities are simply not producing enough high quality graduates. Now this becomes a chicken and egg argument - as I know many co-workers who encourage their kids to do law or medicine or economics "because that's where the money is". People here in the USA need to remember why this country became as big and great as it is - before the Chinese take over - they do know how to produce large numbers of science garduates. Just look at the post doc populations in US universities.

Americans do not want to go into science because it looks more appealing to them to make money in marketing and financial speculations. This seems to be more fundamental reason for the low number of American post-docs compared to foreign ones. Foreign post-docs are much more likely to gladly accept lower salaries than Americans, because in many cases these "low salaries" by American standards are actually higher salaries by a foreign post-docs' standards (compared, of course to the one's previous experience outside of the US). Only few years later it actually sinks in that the pay was low for the living expenses.
Americans do have a choice to go into much higher paying jobs in marketing in financial; well educated foreigns often don't have such a choice. So, making a living in the US by doing science seems to be very appealing and relatively easy to well educated foreigns (because they are good at it anyway).
Maybe now, after well deserved financial bust, public opinion of science and technology, and jobs that actually produce something (products or knowledge) will finally rise, and doing research will become attractive again more than Wall Street speculations.

American scientists have needed support for a while! The quality of American students is fine when you level the playing field. The foreign students often pass down tests, labs, and homeworks which gives them an unfair advantage over their American colleagues. I have been in compbio/bioinformatics courses where out of several foreign students from a given ethnicity they would get the answers from one/two of their country men/woman. So, they all get A's while the few American students get B/B+'s.

I have also worked in NIH labs where Foreign PI's would only hire people of their ethinicity and then would get pissed when they were told that they need to diversify their group . Imagine an American trying to do that in a foreign lab?

Pay American postdocs and scientist to go into science or like the person said China/India will surpass the US.

I agree that the number of foreign post-docs drives down salaries. India and China academic research professors made $10-12,000/year, so accepting 3x that per year to live in a country with luxuries like central heating is very attractive. I don’t think American graduate students are not well trained. We have some of the best science graduate programs in the world. I also don’t agree that American junior scientists are in short supply. In the institutions I worked at, 75-90% of graduate students were US citizens, but 5-16% of the post-docs were US citizens. An analytical person that loves science simply can’t enjoy it enough to legitimize working twice the hours for half the salary, 10% as much job security, no retirement benefits, in some cases no medical benefits, no formal evaluation and promotion structure in place, temporary 1-2 year appointments, and 2-5 years post-graduation date – no career stage specific funding mechanisms to apply for. Grants intended for independently funded scientists require the individual to have their own lab space, but there’s a bit of a catch-22 to getting it since the universities want to hire faculty who are already coming with their own funding. So, a lot of people get stuck in a perpetual post-doc. People with permanent residency have a lot of transferrable skills plus meet the eligibility requirements to be hired into other positions, so they don’t stay stuck as long.