I want to let the community know that there is a fix availble for the issue that is listed in Gemini as
"Redirect to Login page after User Login" (issue id is DNNP-1773).

The issue is that, unlike in versions 1.x and 2.x, if you have a login page and home page designated in site settings, and then a user is forced to login as a result of trying to access a protected page while not already logged in--after they log in, they will be redirected to your designated home page instead of the page they were trying to access before logging in.  Also, even if you don't have a designated login page and home page, if the URL of the protected resource includes query string parameters, those query string parameters will be gone after the login.  They will get to the originally requested page, but any module that required those query string parameters will not work as intended.  The latter case was a problem for me in a site I'm currently developing that has several custom modules that rely on query string parameters.

In the comments for this issue on Gemini, another user, Ken Lyons, listed some code changes to fix this issue in version 4.0.3.  Based on those changes, I've fixed the issue in my installation of version 3.1.1 and posted the exact changes I made in another comment on Gemini.  You can find the issue and the code here: http://support.dotnetnuke.com/issue/ViewIssue.aspx?p=0&id=1773