I am an AA user and have no caps on downloads at all. They do in fact do truly unlimited deals but some info is really, really hard to find - for some reason stuff seems to be missing from their web server.
I can download a literally unlimited amount of stuff, within pragmatic limits: the limits of my line rate times the limited number of hours in the day, and the limited amount of money I have in my bank account. Anyway, I will not get cut off or capped or throttled, I just get charged for what I use in the case of DSL. I pre-purchase a certain amount every month which makes it slightly cheaper per MB download, but due to my utter stupidity I always get it wrong and buy too much so I usually end up wasting a fiver or so.
On AA 3G I can set it up to take certain actions at a quota limit that I choose, so that is slightly different. There I do not pre-purchase anything, I just get charged for what I use.