Incorrect.
As long as the BlackBerry you purchase from the US is unlocked you can get it re-connected in Aus. (Phone and Blackberry services)
However, I'm not sure on Virgin mobile but I think they use Optus as their carrier. If so then they probably offer a BlackBerry plan. (I will check it out and post back)
Obviously, if you purchase a CDMA BlackBerry from OS (ie; 8330 Curve) then that wont work here because of the network infrastructure difference but any of the GSM handsets will. The 8320 that you have listed will have no problems operating of Australian carriers.
Just had a quick look and it looks like you are right in saying Virgin mobile does not offer a BlackBerry plan (or handset). (Seems strange because they buy their bandwidth from Optus). So looks like you might have to look at one of the major carriers.