After completely forgetting about it, the Google Summer of Code T-Shirt has finally arrived. I participated as a code review mentor for Drupal on three student project.
The modules I reviewed were Angela Byron's Quiz module, Fabiano Sant'Ana's WebDAV module, and Matthew Loar's Google Site Map module.
I use Matthew's module on this web site and others as well.
Back to the T-shirt: it is quite natural that it has the word "summer" on it. It also had a sun-like drawing. Sun and summer are distant memories for me, having just emerged from the third snow storm of the season in Southern Ontario's winter weather.
Update: As per Mostafa's request, I am including pictures of the T-shirt.
As you can see, the "Google" on it is not their corporate logo for some reason.