Matt Sweeney sent me this tip: "Google calendar has recently added compatibility with CalDAV, so Google Calendar/iCal syncing is possible without any middleware." +1 SwampDrain point to Google. The short list of known issues is interesting reading. For example: "It's not possible to browse room availability or reserve conference rooms from within iCal. This feature is not supported by CalDAV."
Update: Further discussion about the ins and outs of this over at The Google Operating System blog.