A couple of days ago Andy Seaborne posted a link to The SPARQL Algebra, "The SPARQL Algebra defines the semantics of a SPARQL query execution. The algebraic expression is derived from a query string by parsing and transforming the abstract syntax tree. The result of a query can then be calculated by the evaluation rules. This gives the correct results of query -- it does not imply the actual execution of a query must be performed in this manner."
This seems influenced by Jorge Pérez's work (or least it shares a common basis as the definitions are very similar) and looks like it's compatible with the stuff that I did (it even has antijoin in there).
And in other SPARQL news, Danny Ayers has published (based on discussions between Max Völkel and Richard Cyganiak) a SPARQL Update Language for insertion and deletion of triples.
Update: Andy Seaborne has posted about his work on this algebra, a version of ARQ is available which implements this algebra (it's post 1.4). The use of the FILTER command is also discussed. Via SPARQL will be formalized as an algebra.