Ever since updating to Mountain Lion I've not been able to login to https://bp.ato.gov.au - the ATO Business Portal. Turns out you that the authentication is done via a Java applet (using SAML tokens) and that upgrading to the latest OS ended up with no Java installed and thus no access to the portal.
The easiest way to get Java back on Mountain Lion is to fire up a local Java based application which will prompt you to download and install Java. For me, that app was Open Project, a Java based project management tool. Once the install finished restart any browsers for it to pick up that wholesome Java applet goodness (end sarcasm).