Thanks to Bioepi for the comments and vid.

I purchased magicsim 23th-A on ebay and have set up using Nokia N81 as per Bioepi's instructions. In the N900 it does switch sim's and connect to to Telstra and Optus GSM networks in Australia, although I can confirm that it does NOT work with Optus' 3G network, though I guess it is possible that the adaptor I purchased is faulty.

For those wanting to give this a crack but are put off by cutting sim card's - don't be. I was initially nervous about doing this but found it quite easy.