Pod2g has posted instructions on how to get a working dnu debugger on iOS versions greater than 4.3.
People know that the gdb package coming from Cydia is broken since 4.3. But here is a simple way to have a working gdb running on your iOS device : use the one from the Apple SDK !
You can find his instructions on moving over gdb from the SDK to your jailbroken device at the link below. It's fairly simple.