There's a discussion going on over in the WPCentral Forums concerning Windows Phone 7 apps and what to do if you accidentally delete one from your phone.  Unlike Windows Mobile, there isn't a centralize listing of the Marketplace apps your purchased to re-install from. This doesn't mean all is lost.

You'll need to access the Marketplace from your Windows Phone. From there all you need to do is go into the Marketplace, find the app you need to re-install and go through the installation/purchase steps. Instead of choosing how to pay for the app, you'll be notified, "You've already purchased this app. Would you like to install it again?".

Just tap "install" and you'll re-install the app.  You'll find the full discussion over here in the forums.