Gabe Jacobs made this video of dragging and dropping iPhone apps, which proves conclusively that you can drag and drop iPhone apps you download from the internet into iTunes. What it doesn't prove is whether or not it will work on your iPhone—it almost certainly won't—since apps from the App Store are wrapped in DRM.
If it works like iPod games and iTunes music, you'll be able to mail them around just fine, but once you get around to actually playing them, it'll prompt you for the correct iTunes account tied to this piece of software. On the other hand, if you can somehow strip the Fairplay DRM off of it, that means free Apps. We know they exist for music, but we're not sure about iPod games. Does this also mean that if people create games without DRM, you'll be able to directly import them into iTunes, bypassing the store entirely? [Gabe Jacobs via Gadgets on the Go]