I have exactly the same problem...Since i've upgraded to 1.1.2 my voicemail button doesn't seem to function properly. Back in 1.1.1 every time I received voicemail a simple press would direct me to my voicemail and I would check messages normally. Now everytime i press the voicemail button it doesn't respond on the first press and on the second press it just highlights and does nothing. I installed iWorld and it only fixed the VM notification problem, which i didn't really care for since the red dot notification was enough for me.
Is there anyway to fix this problem so that it goes directly to my voicemail whenever i press the voicemail button?