I can confirm that Imaginet has had this in place for a little while as well. I currently have my account set up such that my mum just uses the same account as me since I rarely hit my cap and she hardly uses much bandwidth.
I remember the original Marconi modems that Telkom supplied with ADSL, and their nasty default security settings. Remote connections were allowed and the modem had a standard username and password... Was absolutely pathetic. Nevermind that fact, it was an awful awful device that hung continuously... but anyway, I digress.
Dave, you'll have to contact your ISP (not SAIX) who will reissue a new one for you - I don't think you'll have the luxury of picking one of your own, but thats fine, probably more secure that way (in case someone sniffs your "standard" password from some other service). I suspect that they will probably require some sort of identity confirmation, possibly even to the extreme of faxing them a copy of your ID book.