There are three reasons why this might be:
#1 You have subaccounts
As we mentioned in the release notes, the first stage of deployment is to move across all customers who do not have sub accounts.
So if you have subaccounts, you'll need to be a tad patient and have your eyes peeled at the beginning of 2013.
#2 Login via old URL
If you bookmarked the previous login URL (extranet.smscentral.com.au) and you are trying to login through that page, you'll keep seeing the old interface.
Simply change the bookmark to my.smscentral.com.au and voila.
#3 Empty the cache
Yeah, that's the simple answer to everything, but sometimes your browser might not be updating info as fast as we are (especially with older browsers).
If this is the case, just go to your browser's tools and select Clear Cache.