This Wired article summarizes and links to a poster for the American Geophysical Union meetings (pdf) from Elaine Hart, a graduate student in civil and environmental engineering at Stanford. Her power flow simulation suggests that the existing transmission network in California can accommodate up to 70% of renewables in the portfolio on a hot summer day. The number of overloaded lines in the simulation rises from 11 to 31, which is not that large an increase given that there are almost 5,000 transmission lines in California. Still, this kind of work can be really useful to help target transmission investment.
The Wired article also has some good links for further reading. I look forward to seeing more of this research!