FSP's post yesterday reminded me of my own often absent (and finally, retiring) advisor. I've mentioned him in passing several times, and in this post I touched on his frequent disappearances.
My advisor was on sabbatical for part of my time at grad school, but it didn't have much of an impact on my academic career or progress toward completion. I think he took a 6-month sabbatical right before I started most of the fieldwork for my thesis. So he was around to help me start out, and he was more or less available to help me with my thesis. It probably helped that he kept my university town as his primary residence during the sabbatical, so he was only slightly less available than at other times.
So what did my advisor do for his sabbatical? Well, he taught several short courses in various subtropical areas that were close to beaches. He renewed his contacts with his massive network of colleagues and ex-students by visiting just about every major department doing work in our particular subfield. He attended conferences in exotic locations (somehow, I was the one who went to Ohio...). And twice a week or so, he would read his e-mail and help out with whatever crises were simmering along in his absence.
Did I feel neglected during this time? Not really. I knew what I was getting into when I applied to work with him for my master's degree. He was close to retirement, didn't have anything left to prove, and was upfront about being fairly hands-off. I was probably more motivated to finish than your average grad student, since I had a sweetie waiting for me several hundred miles away.
If I had been a needier student, if my advisor had shut off communication for 6 months, or if he had taken off during a time I had needed more attention, maybe his sabbatical would have been an issue. As it was, I just told him to have a silly drink for me and that we'd reconnect once he was back. My only regret is that I can't have a sabbatical myself...