The basic setup is pretty simple, i think it wouldn't take too long to rewrite it in Ruby on Rails. Perhaps it is more complicated than it seems. The main reason for rewriting it would be to make it open, figure out a way to have it both provide centralized topics / tags while decentralizing the servers to allow for scaleability. The latter is less important than the openness issues. A third thing, is if you had it open, you'd provide more ways for exploring mapping tags to topic graphs and ad-hoc hierarchies. This would open up the possibility of solving some of kellan's issues with tags and information architecture.
Blaine's been talking about his explorations of rdf. He's got an interesting set of libraries which rdf quires on to mysql tables. Perhaps something like that could be extended to make a topic map like system for exchanging meta information in the form of link tags.