O.K. So this is a masked answer in a question.
I found my answer and I thought I would share so that it would help others who may be trapped in iPhone app deployment hell, especially on iTunes for Windows.
When attempting to install an app via AdHoc or Enterprise distribution via iTunes for Windows I would get the error 0xE8008017 "unable to validate signature".
It turns out the cause was a hidden Thumbs.db file that Windows dumped into the .app folder / bundle.
To fix: - Remove the app from iTunes and send it to the Recycle Bin.
Go to your source folder for your app.
Make sure that "Hide operating system files" is unchecked in your Windows Explorer view preferences.
Go into the .app folder / bundle.
Delete Thumbs.db from your app bundle.
Re-drag and drop the Thumbs.db file into iTunes
Sync, and all should be well.
The overall issue is that Windows and Mac have different file names, and do random file drops into folders. If so much as a single file name or additional file is added to the bundle it will fail the iTunes install hash and check.
Good luck, --Batgar