Here it is:
https://www.google.com/calendar/images/calendar_sm2_en.gif

Looks pretty nice, doesn't it?

Oops .. wrong URL. Here's the real thing:
http://www.google.com/calendar/images/calendar_plus_en.gif

And the old one:
http://www.google.com/calendar/images/ext/gc_button1_en.gif

Matches with the "Add to Google" button at rss feeds

And the rest of the "Add to ..." things around there
Nice :)