Launch app from market
When searching in the market you should be able to open the app if already installed.
Simon Plex commented
You almost can. Almost.
Tap on the name of the app from the store and you will be taken in the app list with the said app in the middle of the screen. Why this doesn't happen on the icon, or why it doesn't just start is beyond me.
Android can open any app already installed when searching from the market. WP7 just says you have it installed. Just add a button to launch it.