In computer science - my field - a lot of graduate students are foreign, yes. I don't know a single one who plans to "go home" when he's done. Most of them aren't getting financial support, either. Their tuition helps pay for my assistantship.
So when we're done, I'll be competing with "foreigners", yes; but foreigners who have been in this country for years and are largely acclimated, who want to work for the same wage, not less. And apparently there's no lack of jobs for CS Ph.Ds - I understand the starting salary right now is around $90,000 - that's not an indicator of an overfed market.
no way are there large numbers of CS PHd entry level jobs at $90K floating around out there. you are dreaming. its becoming a niche market in the US, the top graduates from the top schools like MIT may be getting those jobs - but few others are.
True -- one of the bright spots in science. But starting academic salaries in most bio and chem is similiar to math, in the 40s. Another hot area is bioinformatics, and more generally, statistics.
Are there any postdocs in your department? If so, you might ask them what they are finding the job market is like for recent PhDs in computer science.
I suspect that they won't tell you they are making $90,000 in their current position while they are trying to pick out which school to teach for.
Instead, I think you will hear about the multiplicity of candidates for each advertised tenured position, and the brutal competition out there for these positions. There is a recent trend toward offering non-tenured teaching position (adjuncts and such), but the pay for these from what I have seen is not anywhere near $90,000.
But let me know if you find differently; I would be interested in hearing about a strong job market in this area.
Best of luck with your career choice.