i don't think that the rate of change is different.. i think that what has happened is we are aware of all the changes, in all the dialects..
Scot's english is so different than the english of london (less so now, than 100 years ago.)
English in US is more unified too.. Newspapers, transportation, wars, TV and the internet have served to level the language.
The local differences (that 1000 years past resulted in vulgar latin becoming French, or Spanish, or Italian, or Romanian) are less localized.
I not only read about slang in Oz or NZ, i socialize (albeit infrequently) with friends from Oz or NZ, or Scotland, or South Africa.. and i learn their slang, their lexis, and while i don't use it--occationally a word comes along, that is just the right thing--and that word gets a much wider audience than ever before.
many computer users all over the world use google --it borders on defining a search engine.
100-150 years ago, chains (like woolworths, like A & P, like Sears (& Roebuck)) were becoming well known, not just in 1 place but in many--the created a world wide standard for shopping.
the same universality is now extending to language.
