I bet they want to steal this hobby electronics market and make it into its own pet.
I'll take that bet.
What I think is that Google wants to make the Android phone your interface to everything digital: the web, your music collection, your home entertainment system, and all the networked goodies like smart appliances, home heating/cooling controls, and burglar alarms, that are arriving in the homes of industrialized-nation dwellers.
They see the Arduino out there doing for PCs many of the same physical-computing interface tasks they'd like to do with Android. It's got a great established base of peripherals, tutorials, examples, and user communities.
So all they need to do is write a little glue code, and their users can immediately, and easily, do all sorts of "way kewl" stuff with their Androids by leveraging existing Arduino experience.
I do see some risk that Google could hurt vendors of Mega boards, especially if they subsidize the ADK boards to encourage adoption. Over the long run, though, I think it's most likely that the ADK will expand the market, creating a rising tide that will help raise some of the existing vendor "boats", while increasing volumes and lowering prices on many add-ons. And maybe bring us something really interesting: a low-cost Arduino with a built-in Bluetooth interface, and a standard protocol to simplify project like wirelessly-controlled robots and kinetic art.