Kevin at Web Analytics Island uses the user agent string
Mozilla/5.0 (en-us) ApplewebKit/525.13 (KHTML, like Gecko; google web preview) Version/3.1 Safari/525.13
to identify the Google bot, but this information (as a complete string) is not available in Google Analytics. But I could check that visits using the Google bot version of safari increased after the introduction of Site Preview:
This is kind of what I wanted to see.
I made an advanced segment to contain this data:
Looking at the “Service Providers” report shows that this segment is definitely interesting, even if it is not tracking the Google Site Preview Bot: