Xcode 4 says "finished running <my app>" on the targeted device -- Nothing happens

For those reading this in regards to Xcode 4.2, and attempting to run on an earlier device (e.g. iPhone 3G, 2G, iPod 1st gen, etc) I have another solution. New projects created in Xcode 4.2 by default specify 'armv7' in the 'Required Device Capabilities'. You'll need to remove this if wanting to support devices that run armv6 (e.g. the iPhone 3G).

Delete armv7 from the 'Required device capabilities' in yourProjectName-Info.plist

You may also need to change the build settings to compile with armv6 instead of armv7.

This is the default:

Double click on 'Standard (armv7)' to add another, then click the '+' in the popup, and type in 'armv6':

Click done and it should look like this:

I just had this problem, but for me it was because I only specified armv7 in my architectures, but the 3G is armv6. Make sure you have both.

I had this issue, and it turns out that XCode was defaulting the Deployment Target to 4.3, whereas my phone is 4.2.1. Changing the Deployment Target to 4.0 seemed resolve it.