I see it like this:

Android is the Open Source alternative to Windows Mobile, a generic OS you can just slap on any device for a decent UI and many applications. Symbian ranges in this class as well.

Maemo is (going to be) the Open Source alternative to the iPhone OS or Web OS, a system that is highly tailored towards a specific range of high-end devices. It may be less ubiquitous (now and in the future), but is more likely to provide a pleasant and unique user experience.