Eventually I decided I would reduce the targetSdkVersion down to 8 and see if I could build and run on the old device. I updated the manifest and did a clean build for good measure. Eclipse built my project and ran it on the device. Android Device Chooser was now happily insisting that I should Select a device with min API level 8.
Yet another example, and a very good reason to set this attribute, is the move to sd functionality. This was added in API level 8 (2.2). If your android:minSdkVersion is set to API level 7, and android:targetSdkVersion is not defined, devices which run API level 8 and up would not be able to use this function.
That, at least, is a more reasonable error. It means that your app has a of Android v2.2, which is API level 8, but you don't have the API level 8 SDK installed.
I believe Android API level 8 was dropped starting with Unity 4.5, which made it unlikely to be able to continue publishing Unity developed apps on the Nook app store. Any time I submit an app now is rejected because they still use Android API level 8. Now that I'm using Unity 4.6, I can't even revert to an earlier version of Unity and compile for Android API 8. Is this situation a problem for anyone else? Just curious.