In a discussion about OOOOOOOOXML (or whatever it's called this week), Sam puts some wood behind the definition-arrow for "open standard". He says, correctly IMO :

I prefer simpler definitions. A standard is one that has multiple, inter-operable, independent implementations. An open standard, at least in the software world, is one where at least one of those implementations is open source.

I'd like to augment this by pointing out that the term "open source" implies the usual set of freedoms that accompany the copyright license, such as freedom to use in whatever context an end-user wishes without requirements to engage in a transaction or contract of any sort with any other party. Therefore, an open standard must allow such freedoms for users of open source implementations. If not, the implementation isn't open source, and therefore, according to Sam, not an open standard.