A 2007 study by Vivek Wadhwa et al. looked at the proportion of high-tech startups founded from 1995 to 2005 that had at least one key immigrant founder. The methodology has been replicated to look at high-tech startups founded between 2006 and 2012 and has resulted in a new report released today (report here; press release summary here). I'm travelling at the moment and have not had much time to review but the key finding is a slight drop. 25.3 of high-tech startups had at least one key immigrant founder in high-tech startups from 1995 to 2005; the stat is now 24.2 percent for startups founded from 2006 to 2012. This is more pronounced in Silicon Valley, where the drop went from 52.4 to 43.9 percent, which I imagine will be big news, but I'll part with the reminder to everyone that entrepreneurial activity does happen outside of California.
If you check out the press release, you'll see that Vivek Wadhwa also has a new book with an online resource, http://immigrantexodus.com/.
To quote a couple of colleagues at Kauffman, it appears as though 2013 will be a year of (deservedly) heightened focus on high-skill immigration and entrepreneurship.