Will the new Metro API support in app purchases? We will be selling an app in the Microsoft Marketplace that requires a monthly subscription. Is there a model for doing this in place?
You can use the Windows Store In App purchase, or your own purchase model -- unlike the apple app store, you aren't mandated to use Microsofts IAP. However, there are restrictions such as being PCI compliant.
A good launching point for understanding the options is the windows store blog: http://blogs.msdn.com/b/windowsstore/archive/2012/07/20/making-money-with-your-apps-through-the-windows-store.aspx
For auto renew options, your gonna have to use PayPal, or stripe. Stripe appears to be the simplest API to work with.