I've released version 1.1.0 of Postmarkup. The main improvements are that it generates cleaner html, that doesn't include invisible div sections, and that it should also generate XHTML that validates (previous versions could put block tags inside inline tags for some bbcode).
The basic interface is the same, so it should be a drop in replacement for most people. The base class for tags has changed (but not significantly), so if you have any custom tags, it will require a little work to get them functional again. See postmarkup.py for example tags.
I actually quite like bbcode as a light-weight markup. Possibly I'm rationalising, as I've spent a couple of days polishing it when I should be job-hunting! Its great for its original purpose, of letting forum users format text, but it could be used as a more friendly alternative to XML. It's also higher level than (X)HTML in that a tag can generate intelligent content rather that simply applying a style.
Now I have to find some other project to keep me entertained before I re-join the rat race!