I'm only slightly hesitant about this, as style/coding conventions vary a lot between companies/organizations/projects. The question remains: if it hasn't been configured, is it better to fail, or use a default configuration (which will be "right" some percentage of the time, and "wrong" the rest, then requiring the user to configure it). Overall, I'm +1.
I'm not sure I see the need for special support for "checkstyle.config = 'google'" or "checkstyle.config = 'sun'". Assuming that the config setting goes to something equivalent to the checkstyle ant task's config attribute, it says it supports resources from the classpath, so using "sun_checks.xml" or "google_checks.xml" should work out of the box. Instead of having special settings, call it out in the documentation that you can use it that way... and that leads to people being able to use new default styles without further code change, including styles that the user adds in via dependencies in the "checkstyle" configuration.
Using google as default seems reasonable to me.