In the Beach theme there's Nessie, the Loch Ness monster:

And the "Sweet dreams" theme has a nice Pi:

Tea house:

Seasonal scape:

City scape:

For a way to see these easter eggs, check Ben's comment. You'll have to select a theme and inject the content from the CSS file corresponding to your theme:
http://www.google.com/ig/skins/NAME/NAME_3.14am.css
where NAME is in { teahouse, cityscape, beach, sweetdreams, winterscape }
* As you might know, pi=3.14159...