I've done some work lately to figure out how to track how people end up at your Android market app. In a nutshell, you craft Android Market links and the information encoded in those links is available in your application.
Scan the QR code below to get my demo app and you will notice that it knows that you found the app from my blog.
For contrast, uninstall the app and search the Android Market for it manually. When installed it will not show any referral information.
Once setup in an app, it is possible to optimize marketing the app. Simply add some telemetry or use the premade Google Analytics option, and now you know which traffic sources to concentrate on and which to ignore.
Hell, you could even setup an affiliate program.
For details review the app's source code on github