Telstra Crowd Support recently posted a rundown the Aussie carrier's official position on three Mango-supported features: tethering, group texting and visual voicemail. The post details what features are OEM-based and which are carrier-dependent, and what customers can expect as far as availability goes.
It seems that across all carriers, WiFi tethering is what customers want to know about most. The good news is that HTC and LG are working on software updates for Telstra to push out to enable this feature, hopefully in December. The feature is OEM-dependent and was originally left out of the Mango update so as not to delay the release.
As for Visual Voicemail, Telstra has no plans to enable it for Windows Phone in the near future. This feature "requires more development work and investment than simply flicking a switch," and thus, Telstra will not put the time and money into it. They do say, however, that they "will consider it in the future," but don't hold your breath.
Lastly, is group texting. Much like the scarecrow's brain, it was there all along. Telstra decided to disable it by default to prevent people from accidentally sending a too many texts or texts to people they don't know. If you are a Telstra customer who would like to enable group texting, simply go to Messaging > Settings > Group Texts on your phone.