I've had very good experiences with Spreedly... you'll need to get a payment gateway, but they handle all the work of renewals, expiration dates, plan upgrading

I've had very good experiences with Spreedly... you'll need to get a payment gateway, but they handle all the work of renewals, expiration dates, plan upgrading/downgrading, etc. They have a RESTful API so you can check status at any time, and will also ping a URL you specify when somebody's status changes so you can clear any cached data. They also do a pretty great job of support via their Get Satisfaction site and via e-mail.