A featured WebmasterWorld thread has examples of issues with Google possibly disobeying the robots.txt file. GoogleGuy and VanessaFox both come in, to offer some guidance on the perceived issue.
The rule of thumb I always use is "the most specific directive applies." So if you say "Everyone in the room, leave. g1smd, please stay; we need to chat" then everyone but g1msd would mosey.
Vanessa Fox from Google offers some useful links to help documentation and tools offered by Google to help you predict Google's behavior towards a specific robots.txt file prior to going live with it.
Forum discussion at WebmasterWorld.