According to a note on the developer’s site, he’s working on it now.
In the latest SVN this plugin tries to call register_taxonomy() before init and this causes an error because the $wp object hasn’t been initialised. Should be easy to fix by moving initialisation into “init”.
I just posted a description of a fix for 2.5RC2 in the Google Code issue tracker.
http://code.google.com/p/simple-tags/issues/detail?id=13
Take the latest version 1.5 !
