When I bought the Streak at Optus, it was already unlocked because with the Streak, I could use my existing Three Mobile sim card on it straight away.
Before I bought it at the Optus shop, the shop assistant inserted my Three mobile sim card into a Streak that was on display at the shop and the Streak was working fine. The assistant confirmed that the Streak was not locked and I signed up on two year contract at the shop.
On my Streak, I tried "Setting-->About Device-->System updates". It opens "http://mobileupdate.dell.com/uptodate/uptodte.html" and shows a message "No software updates available at this time".
I suspect other Australian Streak users who bought from Optus will be having the same problem.
Hope we can upgrade to Froyo soon. Thanks for your help.