OpenURL 1.0 (Z39.88)
The OpenURL Briefly Explained
An OpenURL consists of two independent parts: the ContextObject (or the bibliographic metadata surrounding a citation) and the location of resolver to parse the metadata and present contextual services based on said metadata. The problem is that the term "OpenURL" is also used as a catch-all for all of the independent parts and how they work. This is mainly because it's a catchier term than "Z39.88", which is the NISO standard all this is based upon.
The most common representation of the OpenURL ContextObject is seen as arguments in a URL string (which is referred to as "San Antonio Profile 1" -- more commonly SAP1 -- and is represented in Key Encoded Values -- KEVs). This "representation" is independent of the ContextObject (from here on known as CO) itself and is only intended to permit the CO to be transmitted via an HTTP GET request.
There is also SAP2, which is an XML representation of the CO (see: here for more information) and is a much more human readable format. This still falls outside the scope of microformats, but makes the point that encoding has nothing to do with the CO itself. They are just agreed upon means of conveying the CO to enable machines act upon them consistently.
The ContextObject could be conveyed just as easily in XHTML using attributes, as long as the terms follow the vocabulary defined in the OpenURL framework. The important thing to focus on here is the ContextObject -- the address of the link resolver is institution-specific and should be handled by a user's (or machine's) activating agent.
However, the link resolver is still a very important component to this whole process. Getting users "appropriate copy" is a very real (and very difficult) problem that libraries are trying to solve. Link resolvers are a pretty efficient means of overcoming this hurdle, so it would make sense to mark up bibiographic citations in a way that link resolvers can easily parse.