Currently party 0 is named "Unregistered Visitor". I thought about making it a user for exactly this reason, i.e. allowing for anonymous postings.
I didn't do it at the time because I didn't have time to work out details such as those mentioned by Rocael. Can I block the user data page by permissions at install time or do I need to do extra work? That kinda thang. We obviously don't want to want everyone and his sister modifying a dummy user account.
Glad to hear things seem to work when you make party 0 a "real user" (you should change the object_type for acs_objects(0) to "user", too.)
If you have time to explore things further to see what's needed (if anything) to prevent non-logged-in users from mucking around the user info page etc that would be really cool. If you can verify things are OK we can look into doing this for 5.0, especially if you provide patches for any tweaks you need to do.
You'd need to check the admin pages, too ... Unregistered Visitor shouldn't show up because it doesn't belong to Registered Users. This is probably proper behavior (?)
The actual datamodel and upgrade scripts are trivial so these aren't big deals. We just want to make sure things work logically before taking the plunge.