I ran into a problem using Secure Pages with multiple subdomains. Secure Pages only seems to allow you to put one domain for SSL and not secure pages. I found the following module in Drupal 6, but I'm wondering if there is another project people know about to solve this problem in Drupal 7?
You can actually just leave the base URL and Secure URL blank to not have it redirect to an inappropriate subdomain than you're on currently.
To restate my problem, I had multiple subdomains sharing one database where the Secure Pages module was implemented. If I put down a base and secure URL using one subdomain, I would get redirected to that instance from other subdomains not listed there when https was needed for that subdomain.
This is an undocumented solution as far as I'm aware, so I hope it helps someone else out.