what separates the U.S. from Britain linguistically?

[patrick1]

False.  Mass media is not accelerating linguistic convergence.  Regional dialects may be getting somewhat less apparent in the UK, but that's largely a result of increased geographic mobility.  Meanwhile, dialects are becoming increasingly more distinct in the US, by and large, with the continued decrease in speakers of some East Coast dialects (e.g., Charleston) being more than balanced by large changes in the Rust Belt and California.

I was under the impression that TV (and to a lesser extent, films) desire to have most "normal" characters speak with a flat, midwestern accent was having a rather large impact on our "lesser" accents.  Hence why people in California and the Rust Belt now sound like they just got off the bus from Des Moines.

The linguist Labov showed that, counter-intuitively, accents were becoming more defined. They are fluid and changing, but I think he showed a diverging pattern.

Here is a link to some of his work.  He was featured in a pretty good PBS documentary on American accents
http://www.ling.upenn.edu/~wlabov/home.html