I'm running Exchange 2010 SP1. I have internet publishing for calendars setup and the ability to "publish calendar" is available to the users with the appropriate sharing policy.
I have published calendars with both Public and Restricted access levels. On both sets, I sent the internet share links to myself and have tried to access them from outside the corporate network. When attempting to use any of the links (either HTML web view or ICS subscription) for either of the calendars (Public with its readable URL or Restricted with the obscure URL) I am taken to the OWA login form.
The login form URL does show the redirect target of the calendar page, but that completely obliterates the desire to be able to publicly share these calendars. We want anonymous internet users to be able to open an ICS file and subscribe to our calendar.
How can I get the published calendars to just open without first dropping the internet user into our Exchange server's OWA login page?
As you've discovered from my comment, if you have a catch-all redirect in IIS that is grabbing HTTP requests and redirecting them to SSL (often done for OWA so that the user doesn't get a 404 on the HTTP version), you can experience the issue you are having.
You may check and make sure you are doing proper http/https redirecting instead of a catch-all for everything. http://support.microsoft.com/kb/975341 -- I've seen cases where they've carried over the old Exch2k3 way of using a redirect file but it's better to do it this way.
If you have any questions let me know.