When Simple:Press 5.0 releases, it will require WordPress version 3.2.
This is primarily due to matching php and mysql version 5 requirements… php needs to be at least 5.2.4 and mysql needs to be at least 5.0.15…
WordPress version 3.2 is currently in release candidate so its pretty stable and we have been using it for months. We have not tested 5.0 on version 3.1.3 but it should work.

