It doesn't happen very often, because I try and be conscientious about it: while it's true that WoW is a game, it is (for me at least) a social game and I consider my guildmates friendly acquaintances at the very least. Just as I wouldn't bail on friends when we have an appointment to go out bowling or for pizza or when I was in a weekly D&D game, I don't like it at all when I have to step out of a raid or bail on an appointed "help me get my Northrend Dungeon Hero achievement" or what have you.
Sometimes it's unavoidable. Recently I had to leave a raid before it was done because I simply couldn't stay focused on what we were doing due to feeling feverish and run down. I enjoy tanking and healing with my guild but my health and real life have to come first so I bowed out once I was sure a suitable replacement was available. I've been with this guild since the launch of Wrath now, and we've seen pretty much the whole of the xpac together.
Where do you stand on it? Do you see WoW as something easily skipped out on, or are you in a tight knit social guild where you feel obliged to show up?