Australia started out without caps, then Telstra added draconian caps (3gig/mth with humongous excess fees and a high standard fee), then eventually it all got relaxed to the point now where a good ISP will offer a softcap. The softcaps involve throttling download speed if you exceed, and the caps are definitely high enough that a normal user will struggle to reach them (especially for those ISP's who offer separate peak and offpeak softcaps).

If the US chooses to go for the draconian hardcaps with excess fees route, they're going to be a good long way behind the times and need to take a good hard look at what happened in Australia. Telstra's actions were the trigger to their virtual monopoly crumbling over a period of time.