Michael --
Although somewhat controversial in the Moodle community (because it's a commercial app), we're using aMember to facilitate "subscriptions" for one of our clients.
Out-of-the-box, it requires some minimal surgery to get it working with Moodle...but we accomplished the integration just this week.
Download Grant Swaim's excellent documentation and scripts found elsewhere in this forum
Then, go download and install aMember
Then, go to this thread and download our modified enrol.php file.
Your mileage may vary, so good luck!