[Note: coincidence! I wrote this over the weekend, but have found myself "scooped" by a post on the same theme at the Complexity blog...]
It seems, more and more, I'm expected to miss class.
Obviously, now and again I have to miss class for a conference or workshop to go give a talk. Somewhere I'm sure there are rules and regulations (that generally go ignored) about this sort of thing, but I'm sure my employer and I share the understanding that I'm supposed to go present my research, and it's fine for me to miss a few lectures a semester for that purpose (while trying to get another faculty member or a graduate student to cover, of course).
I'm a little less clear on some of the other options that come my way. NSF panels. NSF workshops. (The FIND grant comes with 3 workshops per year.) DARPA ISAT meetings. PC meetings. Meetings to talk about Visions and Funding and such. Other invited talks. And so on. The underlying problem, of course, is that these things often require travel, which means missing class, which (along with family issues) is one of the reasons I don't understand why people don't think harder about how to avoid travel for these things.
How many classes is it OK to miss in a semester? Offhand, I'm thinking 4 is a goal for a maximum (for my standard Tu-Th teaching), perhaps because it looks like that's how many I'll miss this semester (cancelling 1 class, having 3 lectures covered by graduate students), and that's probably the most I've ever missed in a semester. (There was that one time, I missed the first day of class for the semester, because my wife inconveniently chose that day to birth our second child... but I digress.)
This semester, of course, I got myself in a bind by going to New Zealand, eating up several of my days. But I've definitely noticed more pressure the last few years for events that would cause me to miss class, which, ostensibly, is supposed to be a high priority for my job.