Certainly in recent years airlines have been issuing info on DVT in their seat back mags, some in their pre flight safety videos (on long haul flights), many on their web pages and I've heard cabin crew advise certain people to get up and walk around.
I always make sure I stretch my legs regularly on long hauls but, as far as I'm concerned, as soon as the seat belt sign goes on, I'm back to the seat.
I had a conversation some time ago with a pilot when DVT first became an issue and we talked about walking around . He said it could be a nightmare on an A380 if the bulk of the pax decided to stroll around at the same time. Apart from the congestion, there was the little problem of trim. This was at the time when the type was being touted as able to offer flying gyms, casinos and a host of other diversions and was expected to be in service in numbers in time for the Olympics in 2008. We started to speculate - as you do over a couple of pints..
We decided the only answer was to split the aircraft into sections and have Chinese style excercises led by specially trained crew - perhaps Virgin would take the lead with lycra clad section leaders.