Ever since upgrading to the 4.1 SDK my Entitlements are no longer valid when users try to install the Ad-Hoc builds. We originally had this issue a long time ago and and solved it by adding to the project an Entitlements.plist file with a get-task-allow = false key.
I don't think I've changed anything in the project configuration except upgrade the Base SDK to "iOS Device 4.1". I've also tried removing and creating a new entitlements file with no luck.
If you are using Development profile then you have to set the value of the Entitlement to true. But if you are using Distribution profile then the value should be false.
I tried this and it worked for me.
Here's a working Entitlements file screenshot: 
The Code Signing Entitlements key in the project settings should be blank.
Be sure that your setting for code signing identity is the same for Target and Project.
I had a problem of getting "Signer is not valid" error when installing through other iTunes than the one on the development mac. Very odd, but there you go.
Target is reached by right-clicking your app name under targets in the left menu and 'get info'->build check all three configurations, debug/release/ad hoc they are not necessarily the same, but they have to be the same as in Project (target:debug as project:debug and so on), which is reached by using the top menu, "Project"->edit project settings->build
Having all three matching up made my ad hoc builds work again... after much headache and hair pulling. Hope this helps. :)
Try revoking your Ad Hoc mobile provisioning profile and regenerating it. Sounds a bit hacky, I know, but I've had a number of issues with build signing that I had to resolve by nuking everything and starting over from scratch.
After Mucho Frustrationado, I finally (with the help of this thread) got my app working again! :)
Thanks for the tips everyone, especially Nicki!
My situation was that I had an app working on my 3GS, set it aside for a few weeks and when I came back to it, I remembered that I was having trouble. I was merely trying to get a Debug version on my phone, but ran into the situation where it would start to load (you'd catch a glimpse of the Default.png), and then it would quit. The Debugger just said:
Error launching remote program: failed to get the task for process 532. Error launching remote program: failed to get the task for process 532. The program being debugged is not being run. The program being debugged is not being run.
I went through the whole Entitlements quaqmire, and finally, after revoking all my certificates (through Keychain Access) and re-creating all my Provisioning Profiles, Developer Certificate, et al, and today's step which revolved around all kinds of screwed up version values in my Project and Target Build settings.
Today, my last piece of the puzzle, was to get all the versions and Code Signing stuff straightened out as Nicki described. Make sure no "gremlin" settings (values you may have plugged while pulling your hair out) are left behind, check the versions match (the Base SDK you're publishing for) in Project and Target Build, check the Entitlements (I had to manually assign mine as there were multiple Distribution profiles, and Xcode guessed wrong), cross your eyes, and Build and Run on your device.
Thanks again everyone! Hope this little tidbit helps someone else connect the dots. :)
Ciao,
-Jei.