Hi Simon,
There's no "official" logo... officially. But the JSON-stat.org website and the presentations attached to it are consistent in the use of iconography and typography (also in JSON-stat apparel like
).
Because the "corporate" logo uses a regular character with zero customization there are probable no licence issues. The logo is a white "}" written in bold face using the Calibri font. It's suposed to be used on a black background (for example a black circle).
In the website, "JSON-stat" is written using the Nevis typography. This typography is free for personal use but might not be free for other uses.
Xavier