Application not installed because it not signed iPhone 4

Well not sure if this is the same error, but are you running windows 7? if a driver is not digital signed it wont let you install the program. Maybe there is some kind of conflict with ur comp,itunes,and the phone.
Custom Intel Quad Core 2.8GHZ 4 GB Memory Windows 7

Hi Brian,
Its always worth trying to deauthorize and reauthorize your iTunes software. In the Store pulldown menu, choose deauthorize, then reauthorize.
MacBook Air; MacMini; iPod Touch Mac OS X (10.6.1)

Thanks for the suggestions. I am using Vista. I tried deauthorizing and reauthorizing but it didnt help. It was working before just fine until I started having other problems and tried to restore. Now none of the apps will install from iTunes to the phone.
iPhone 4 (for now) iOS 4

I am facing the exact same problem. All the songs on my phone have vanished and I cant add the songs back or add any new apps. I get an error message saying that XYZ application cannot be installed because ABC iPhone is not signed.
I am, however, able to add apps through the iTunes app on my phone. Just not through iTunes on my laptop.
HELP!!!!
Message was edited by: renu19
PC, Windows Vista iOS 4

OK, after a little bit of trial and error, I have discovered what was causing this. The latest update from ZoneAlarm that has updated on Windows Vista was blocking my iPhone from syncing. Turn off ZoneAlarm, or whatever other protection you might have that could be interfering, and the problem should be magically fixed.
I was able to sync previously, but I remembered that ZoneAlarm had updated right around the same time I started to have this issue. I cant believe more people havent run into this. ZoneAlarm is notorious for coming out with an update that screws everything up. Let us know if this solves your issue.
iPhone 4 (for now) iOS 4