An interesting and detailed discussion on various marketing funnels looks at Traditional, Web 2.0 and Holistic methods.
Most of us view conversions within a more restricted scope. And that scope is often prescribed by being one of four basic site types: content (conversion == subscriptions, ads), ecommerce (conversion == sales), lead-generation (conversion == form completions, whitepaper downloads), or self-service (conversion == fewer complaints).
In the end, however, it is up to you to decide what makes a conversion. And of course it is perfectly acceptable to target multiple conversions.
In practice poorly converting buckets outnumber high conversion funnels.
Delve into this one at Cre8asiteforums in Traditional -> Web2 -> Holistic, The Conversion Funnel Mindset