The puzzle with usage caps on Australian broadband has been that it seems divorced from any real cost incurred today and no one had decided to offer them. I had suspected an adverse selection issue whereby the small percent of very high users would migrate to the first mover causing them disproportionately large costs.
AAPT have announced that they will move first. A two-year $100 per month plan on ADSL2+. I can only wish them well but I think one constraint they face is that Australian usage is likely to be higher because our Cable Television options are much poorer. That means TV and likely movie downloads taking up lots of bandwidth. Nonetheless, for those who worry about their monthly usage, I can tell you that not having to worry at all really is worth some extra dollars per month. (And if you are on Telstra BigPond Cable, I think it is well worth saving an extra few dollars per month!)