Android: not as open as you might think. But that might be about to change
The key take-home message is that it's not the software - it's the add-ons, the approval process, and some data assets. Android the operating system is open source. As Andy Rubin memorably tweeted, you can just grab the source code, make whatever changes you like, compile it, and off you go.
But this isn't the case for several key adjacent products. Notably, the Google Mail and Google Maps apps and the Android Market are all highly proprietary, as is the Android compatibility test process. Google uses these as a way of enforcing its terms on Android vendors. In the case in point, Google was trying to prevent other vendors from using Skyhook's database of WLAN hotspots to provide the network-based location service. In order to do this, they threatened to refuse compatibility to them unless they knuckled under.
One of the reasons to do this was, of course, that Android devices help Google add to its database of WLAN hotspots. Not very nice behaviour from the "Don't be Evil" company.
This also demonstrates the thinness of Google's control over Android, though. As far as the Market goes, there are already indie app stores out there. It's not beyond the wit of man to include an alternative e-mail application - Telco 2.0 uses K-9 Mail, an open source fork of the GMail app, for its mobile e-mail - or an alternative mapping and location application. Presumably Microsoft (and Nokia) would be delighted to ship some version of Navteq's mapping technology. Or you could build something that uses the Open Street Map. And of course, once you decide to fork Android you don't need to worry about the Google compatibility tests any more.
Now that Google is competing directly with the operators on payments, how long before we see the first "ForkDroid" phones? Here's a data point: HTC has unlocked the bootloader on its devices, allowing users to (if they want) run their own alternative operating system.