[gchallen]

This varies a lot from faculty to faculty and from position to position. Some will work nonstop regardless of how many grants they have. (If you have grants, use them; if not, write them.) For pre-tenure research faculty, behaving this way is pretty much expected.

Others who are more senior and less research active may shoo their students off to internships (particularly in CS) and spend the summer doing very little. A lot of departmental committee work is paused, and people tend to know who is and is not available over the summer and will make arrangements accordingly. (I.E., don't assign Professor Tee Time to a committee that needs to function over the summer.)