Comcast isn't quite a monopoly, and won't be even if they've merged with Time Warner. That said, the number of choices for cable/internet/phone to a specific person tend to be pretty small ... and sometimes the number of choices is one, but often it's two or three. For example, I live in the suburbs of Austin, and can get service from Time Warner, AT&T, Direct TV and Dish Network. Now, the last two are really only good options for cable and not phone/internet, but even so, there's still two choices for that. And Grande is available in some parts of town (but not where I live), and Google is coming too.
And that said, if enough people get pissed off at a true monopoly, the government has been known to step in and tear them apart. They certainly want to avoid that.