iPhone Debugging: How to resolve 'failed to get the task for process'?

The ad-hoc profile doesn't support debugging. You need to debug with a Development profile, and use the Ad-Hoc profile only for distributing non-debuggable copies.

I have had problems debugging binaries on the device via XCode when the app includes an Entitlements.plist file, which is not necessary to install onto the device for debugging. In general, then, I have included this file for release builds (where it is required for the App Store) and removed it for debugging (so I can debug the app from XCode). That may be your problem here.

Update: As of (at least) August 2010 (iPhone 4.1 SDK) the Entitlements.plist is no longer necessary to include in your application in many cases (e.g., distribution through the App Store.) See here for more information on the cases when Entitlements.plist is required:

IMPORTANT: An Entitlements file is generally only needed when building for Ad Hoc Distribution or enabling Keychain data sharing. If neither of these is true, delete the entry in Code Signing Entitlements. (emphasis mine)