To illustrate with a practical example of how the regulations work, X18+ materials cannot be sold in any states and territories of Australia, except the ACT and Northern Territory. However, the regulation only restricts the sales and sellers, not the purchase and purchasers (with an exception that a minor may be fined if they knowingly purchase adult materials). As such, NT and the ACT have quite a trade going in mail order adult material to other states and territories within Australia, and adults in those locations have no issues with ordering material that cannot otherwise be sold or demonstrated in the places where they live.
The same regulations that govern this material also govern games. In theory (though we suggest you get appropriate legal advice if you are a seller) there seems to be no barrier to simply bypassing the Australian Classification Board with a game and selling it via Steam or Direct2Drive or something similar -- so long as neither the sale nor the seller are taking place within Australian jurisdiction -- and generally they're not. The EA Store probably wouldn't be able to do this, as they do handle their sales within Australia, even if the downloads themselves are coming from overseas.
"Where a sale is within the jurisdiction of the relevant State or Territory legislation," a spokeswoman for the Australian Attorney General's office informed us, "it is a criminal offence under those laws to sell unclassified computer games." -- and of course, the sale via many digital download services does not appear to be under that jurisdiction.
We're not sure if other countries have similar regulations to Australia's in this regard. Games ratings in the USA are completely voluntary so titles there don't technically need to be rated at all, though it is generally done as a matter of course.
All in all, this is a bit of a gray area -- as import and Customs regulations have only partial parity with classifications -- but if it were to prove out, titles that are refused classification in Australia could potentially just move to a digital download system, without significantly sacrificing sales.
Note: If you're looking for the original article on MMOG classification (or lack thereof) in Australia, you can fine that here: http://massively.joystiq.com/2009/01/28/mmogs-defy-classification-in-australia/