You can embed a calendar by clicking on the small arrow next to its name in the left sidebar, selecting "Calendar settings" and then clicking on the blue "HTML" button. A configuration tool will generate the code.
Google slowly rolls out this update, so you may still see the old version that lets you embed only a single calendar at a time.
{ Thank you, Nick Chirchirillo. }
Update (Sept. 6): the feature is live.
