John, it would have to be done manually. To avoid hassle and save time I’d like to mention Luca Trevisan’s great program which converts LaTeX files to wordpress format (it’s used by Terry Tao for instance)
Urs, there’s a difference between a ready-made blog at wordpress.com (such as your new one), and a blog using the software developped by wordpress.org The plugin you mention is for folks who use the software on their own server.
I think it is good that math doesn’t come out perfect here. It should serve to remind you that if you go through the trouble of exposing a nontrivial piece of math discussion, it is a waste to do so just in a blog comment. You should do it on the nLab instead and point people there. ;-)