If it were a major airline I would say go for it. That guaranteed interview (with better-than-average odds of getting hired) would be golden.

But I would not do an unpaid internship at a regional, especially if it delayed my graduation. Regionals are not hiring right now (probably nobody will until summer 2012). When they do start hiring again, they will pretty much hire anyone with a CPL...you don't need an internship. Get a summer job, and spend the money on your ratings.