It seems like Google recommends using ™ for most of their trademarks. Their website says that laws about using ® vary from country to country and there are sometimes harsh penalties for using it with an unregistered trademark. It seems at least OK to use ™ if you are unsure.

http://www.google.com/permissions/trademark/our-trademarks.h...

http://www.google.com/permissions/trademark/rules.html