This brings the known issues for iOS4 to:
The good news is that a fix is coming as explained on the iPhonewzealand site (basically Telecom needs to update its sim cards).
In the meantime XT users can simply set up a contact called Voicemail with the number 083210 and then set this up as a favourite for easy access. Sure not quite as easy as pressing the Voicemail button, but almost.
Btw I also found this EXCELLENT resource on iOS4, which I highly recommend reading. The best summary on the web I have found.
Black and White Version: XT voicemail busted for now under iOS4, but easy fix for now and a permanent one coming.
Hat tip: Image from iphonewzealand