I'm doing mailouts to contacts, with alerts sent to our sales team when they reach a certain score. Although there is a 'lead source' in the contact database in SFDC which I can set to 'email marketing' for example, I don't know of any way to automate the generation of a 'new business lead' or even an opportunity in SFDC from Marketo, so currently I'm relying on the sales team to go from the contact, to the related account and manually create a new opportunity, where they have to manually tag it with the right lead source. Am I missing something?
I'm doing similar with a marketo alert but it's still manual and open to non-compliance. I'm wondering if there is a way to automate it - or maybe this is part of the account based module thats being launched in the Summer (?)
I would use an SFDC Campaign/Marketo program to track the lead's membership in your direct mail initiative.
When the opportunity is created, it will automatically populate the Primary Campaign Source with the most recent campaign the contact has been associated with. Make sure Sales can only create Opptys from the contact layout, the the Contact Roles are enforced.
There are some limits with this to be aware of, worth reading this thread: Campaign Influence and Primary Campaign Source -? - Answers - Salesforce Success Community
In terms of how to create the opportunity, I would agree with Jamie Lewis that a task makes the most sense to prompt sales to act, so that you aren't creating opportunities that Sales does not feel are valid.
If you are sure you want an Oppty and are concerned about sales non-compliance, you can automate the creation of opportunities if you wish -- you need to use an automation tool in SFDC, like Process Builder or Flow. You can also programatically set the lead source of the Oppty at that time.