The interesting thing with using the Trac version in the query string is that:
Users who install from a release package (source or binary) have a version 0.x.y, and the query string change with every release.
Users who install from SVN have a version 0.x.ydev-r1234, and therefore the query string also changes with every update, even of a single revision.
Both Firefox and Chrome correctly cache the files, and re-get them when the version changes. Opera and IE8 display correctly, but I haven't been able to check their caching behavior. I'm pretty confident that it works correctly, though, as WordPress uses the same scheme.