To add to that, I would advise blocking even more than just that domain. I would even advise blocking all third-party static assets hosted on a google/doubleclick/etc. domain. (If there are assets that are needed to prevent breaking functionality such as web fonts or otherwise, then caching them on your own server (or router in the case of personal use) is recommended. This advice might put the reason behind google caching the images in emails into perspective. (google's email image caching tactic wasn't about email marketing, it was about keeping more and more re-targeting data to themselves)
Google Webfonts makes use of user-agent sniffing to determine your OS, and depending on your OS they optimize the font served to the font-renderer in your system.
Interesting. It seems like something that could and should be packaged into a simple JavaScript to reduce chatter and synchronous calls in the JavaScript pipeline rather than having a blocking server call. (So that it can be distributed and cached without upstream requirements)
