xcode build error

I am trying to build an iphone app. Which I have done several times but suddenly I got an error appear i had never seen before and I can't fix it. even if i try to build an app that previously worked i get the same error The error is

_myapp/build/Distribution-iphoneos/BlogReader.app: CSSMERR_TP_NOT_TRUSTED

command /usr/bin/codedesign failed with exit code 1

I have tried downloading the latest Xcode 3.1.4 and latest provisioning profiles, updated my phone to 3.1, checked my keychain so that the Apple. mac developer and my own developer and distribution certificates are in login and system and login is set to default... but still nothing what else can i do?