Not everyone has access to a Mac OS X system. Lots of people are still waiting for critical software to be carbonized to run on Mac OS X, while others are holding on to their legacy machines because they still function quite nicely. However, as time goes by, legacy Macs running Mac OS 9 are being left in the dust; meanwhile, the .Mac services are moving ahead full-steam.
Fortunately, if you're still clinging on to Mac OS 9, you can use an application called Goliath (http://www.webdav.org/goliath) to access iDisks via WebDAV. Goliath can not only be used to access your own iDisk, but to access the Public folders on other .Mac members' iDisks as well.