Hulu is sort of doing this. They have their baseline Hulu+ ($7.99, IIRC). And now an ad-free (for most content, some licensing means there are commercials at the start and end of a few shows) for an extra few dollars. And now you can get Showtime (?) content for an extra few dollars.

Personally, I think it's brilliant (mostly). A multi-tiered platform with expanding, customizable content access. Add in a sports package and a few more premium content channels, and it's what cable should have become for a fraction of the cost.