Brian, currently there is not a way to view multiple calendars at the same time.  However, this is a feature that will be introduced with SmarterMail 14.  As such, I am changing this from a Question thread to an Idea thread and marking it as In Progress.

Brian Henson, the ability to view (and create) multiple calendars has been implemented and is available now in SmarterMail 14, which is now in open BETA.