A popular request is the ability for Outlook to remember what calendars were selected when you last used Outlook and to reselect them at startup.
The good news: Outlook 2007 and 2010 should remember the selected Calendars.
The bad news: it only remembers when you “start in” the Inbox or non-calendar folder.
If the calendar module opens at start, only the default calendar is selected. This includes using the Start in [folder] option or closing Outlook with a second window open to the calendar module. Using Open in new window command will also display just one calendar.
If you want to open to the calendar module and remember which calendars were selected, Public Calendar Choice remembers selected calendars and reselects them for Outlook 2003 and 2007. (It might work with Outlook 2010, I haven’t tried it.)
Public Calendar Choice (freeware)