I updated both iOS 8 and Xcode 6 beta. When I connect my phone, it shows up as \"ineligible device\" where you select which platform you want to run on.
I got stuck on this for forever. Make sure that you didn't choose "Don't Trust" with the computer. If you did, then you will need to make the prompt come up again, and choose "Trust". If your phone is jailbroken, make sure you don't have any tweak (NoAnnoyance for me) that auto chooses "Don't Trust".