Okay good, these replies are making me feel a lot better because I read on some other question and answer sites that professors got mad when grad students took off a few days (!) in the summer to see family, and I was really counting on summer to be able to see my family and participate in family vacation. I completely expect to have to work independently on research stuff during the summer, but it seems like a bit much to expect grad students to be in the lab 24/7 when they're not even getting paid...