Transferring Apps From Your iPhone (or iPod) Back Into iTunes
Ever purchase an app on your iPhone and find out later that it’s magically shown up in iTunes? This is because your iPhone automatically transfers new app purchases during a sync. But what happens if the apps in iTunes are lost? Turns out you can invoke the magic app sync anytime you want. Simply right-click your iPhone in the iTunes Devices section and choose “Transfer Purchases.” Any application on your iPhone that isn’t in iTunes will immediately be moved.
Of course you can re-download any app in iTunes for free, but then you already knew that. ;]