lots of apps now need to interact with other apps. i'll give some examples:
take picture by calling system camera
open url like http://www.shihuangzhilu.com in a browser
android instruments can only test one app per time. can we do automation testing here?
Yes, you can test across applications, see monkeyrunner: http://developer.android.com/guide/developing/tools/monkeyrunner_concepts.html
(not to be confused with Monkey, another android tool)