Amazon App Stores
Here is something I did not expect though... I visited Amazon's app store and installed a couple of apps like "Wifi Analyzer". I still use AppBrain.com to find out if there are newer versions of my apps. It always detects updates faster than Google's Android Market (i.e. Google's App Store).
AppBrain showed me that there was a newer version of "Wifi Analyzer". I clicked to install, AppBrain transferred request to Google's Android Market but installation failed. I checked the error, it was something like ~"bad signature". This is troubling because it means that if I want to have the latest version, I will either have to wait for Amazon App Store to have the updated version, or uninstall it and re-install from Google's App Store.
I guess it makes sense that if you buy an app from Amazon App Store, you cannot use Google's App Store to upgrade it. I wish this was not the case for free apps though. Maybe some day Amazon and Google will work out a deal so that user may use either App Store for upgrades.