I have noticed over the past few days that my text, voicemail and calendar notifications have stopped coming through to my mobile - I kept having to switch them back on only to find some hours later that they had gone off again.
After a bit of Googling I found the culprit to be the Opera Mobile 9.5 beta. When you exit this program, it resets all audible notifications (which the excpetion of incoming call).
The only solution I could find was to uninstall the 9.5 beta and re-install an earlier leaked build which can be found here: http://www.1800pocketpc.com/2008/07/15/opera-v95-build-1522.html/
This build does not display this behaviour and is reportedly just as stable as the official beta.