Archive in xcode 6 is producing a pkg, not ipa
I couldn't find a direct way to do this in the Xcode 6 GM (or betas), but found a couple solutions that have worked:
Archive in Xcode 6 and export in Xcode 5. This worked for me while I was using the betas, but after installing the GM, the Xcode.app binary overwrote my Xcode 5 version (This could be easily reinstalled). @Anan answered that here.
Grab the .app file from your .xarchive and package your own .ipa by creating a Payload directory, copying the .app into it, zipping it, and renaming the extension to .ipa.
I tried using the TestFlight desktop app and dragged the archive into it after creating an account. This seemed to go through the motions of packaging the .ipa and uploading it to the TestFlight servers. I wasn't able to find a trace of the .ipa on my local machine. This probably works, just not sure where to grab the .ipa. You would have to generate the .plist on your own or use an old one.
Regardless, this seems like a bug and these workarounds are annoying to have to do for something that used to take 5 seconds to complete.