Micah has "suggested":http://forums.obsidianportal.com/comments.php?DiscussionID=184&page=1 a way to use OP to indicate players' availability, but that's a somewhat cumbersome solution, and doesn't tell who is available when. Polls on Yahoo Groups are a better way, but they're still not that great for this use, and once closed, players can't change their answers.

I would love to see an aspect of the Calendar feature that allows players to indicate when, on a typical week, they are available to game. This would save me a lot of headaches.