Not at all. If anything, it seems to have accelerated over the course of the past 80.000 Years.
It's what The Economist reports in this article. The original piece of research can be found in PNAS.
Since I don't have time to comment today, just go and read it, it's free.
An interesting personal side-note on the article: it points out how two version of lactose tolerance arose independently in the Indo-European and the African Tutsi populations. So, may be my child(ren) will inherit both versions. I wonder if there are any studies around investigating the effect of the two mutations present at the same time in people's cells. Will they reinforce each other? Or will they have a completely different effect? I guess it depends on the detailed mechanism of action of the two... I'll have a look on the world wide wikipedia...