Shibboleth Identity Provider 3.1.1 Service Update for Windows
The Shibboleth Project has released an update to the IdP 3.1.1 Windows Installer to address a bug that was affecting the upgrade process when the Windows Installer was used in certain cases.
The bug did not require fixing any code, so an updated installer service release was done to make the fixed package available promptly.
Windows service releases are denoted by including a forth version digit in the Windows installer (this particular release happens to be 184.108.40.206) so you can easily tell which version you have. The actual IdP version hasn't changed because the software hasn't.