bbman:
Telstra do have 2100mhz in the City, it was there from before Next G become available. They dont promote it nor do offer the next G services over it, Next G 850 is Telstras 3G network and 2100mhz is really an add on to the 900/1800 GSM network
Remember as well that Telstra's 2100Mhz 3G is part of the 3GIS network which is shared infrastructure between 3 and Telstra. Many of these sites offering 2100Mhz were originally built by 3 not Telstra.
The YHA 3/Vodafone deal in Australia leaves many, many, many unanswered questions at present over the status of joint networks and roaming agreements - potentially some existing roaming deals could now be null and void with uncertainly over 3's use of Next G for roaming and GSM fallback. There is certainly plenty of specultion at present but very few hard facts about what could happen.