The bug has to do with the way the phone software/firmware reads the Micro-Sim and occasionally you may find your iPhone 4s saying No SIM or Invalid SIM or No Service. Often when it displays No Service the only way to get back onto the network is to reboot the iPhone.
This seems to apply to the iOS 5.0.1 (Firmware 9A405). Vodacom South Africa confirmed this bug and requested I upgrade to the latest firmware (9A406). To do that is quite easy, even for novices. Simply connect your iPhone into Itunes, perform a back up, then click the Restore Button which downloads the latest 9A406 firmware to your device. Thereafter you can simply restore your apps and settings from iCloud or from the backup stored locally on your Mac.
UPDATE: A warning to South African users. This firmware update is 849 Mb in size yet Apple does not warn the user that this is how much you are about to download in order to update your iPhone 4s. So please be aware of this, considering our overly inflated bandwidth costs in SA.