Marketing's job doesn't change because the company uses open source software in their products or services to the customer.  Marketing is still there to understand the customer's needs, the market, analyse the competition, get the word out (to build the pipeline), and manage public perceptions about the company.  Lead generation is definitely part of the role, but lead qualification is an ongoing debate between sales and marketing.  

The one metric a lot of people want to believe exists in an open source enabled company is "conversions" of downloads into paying customers.  I believe this number to be a myth at best, and a broken yardstick at worst.  Customers that self-select (qualify themselves) show up without any effort, but they represent a segment of your downloads.  Most people that download the software aren't leads.  You need to structure your website to enable customers to self select out of people that rightly belong in your community.