Ian Davis has been working on URIs that represent places and times.
There are 3 types, a single point in time, a span in time and a place. Interestingly this could be used as a standard format for permalinks. In fact if a weblog entry used an event/author standard for its permalink then it could in theory become truly permanent in that it would be portable.
More interestingly this could extend the use of a permalink for any object within space or time.