When I jailbroke my iPhone 4 with iOS 4.3.1, I used the TinyUmbrella method to preserve the baseband. One drawback of this combination is that the signal bar is always shown as one bar. The iPhone Dev Team released ultrasn0w to unlock your iPhone while fixing the signal bar problem.
There is one side effect of ultrasn0w that I just discovered last week. If you have ultrasn0w installed, the visual voice mail does not work consistently. I was puzzled by why my visual voice mail only worked sometimes. I then tried to unimstall ultrasn0w and found that the visual voice mail started to work once more. However, the signal bar problem came back without ultrsn0w. I did not care about the carrier unlock, but I really wanted the signal bar to show the right signal strength.
I came across this post that solves my problem. The solution is to re-insert the SIM card. One thing to note is that this only works until you shut down your iPhone. If you reboot, you have to re-insert the SIM card again.