My thoughts (apropos of nothing in particular):
It's been coming for a while. The higher speed you subscribe to, the higher your cap. With everyone cutting the cable and going to streaming, it was kind of inevitable.
Presumably, the highest speed would have a somewhat unreachable cap, although I know there are some consumers of data here that will blow away any limits that might be around.
Personally, I have a fairly high tier of Cox services (I get about 150Mb) and I didn't get the email or a letter, but I also go nowhere near my caps (still stream a good bit, but also have DTV).
There's obviously a money-grab component here to compensate for those trading cable tv for internet service, but by the same token, it's reasonable to hold those accountable for their higher bandwidth usage and associated strain on the system. However, I do think with the prevalence of the internet and the ridiculous lobbying of those providers, that ISPs should be regulated like a utility. It's hard to argue that access to the internet isn't a basic requirement of society these days, at the very least equivalent to phone service when it became a utility.