I have noticed the last month time, that my Yahoo and Google visits are going down every day on Yahoo I had no changes in serps, on Google I ad a little slip in rankings from the 6th Aug., but its back to normal the last 3 days.
Still the MSN keeps giving me more visits everyday are people going back to MSN.com for seraches, because the rankings has got better the last 1-2 month.
They are advertising heavily in London? I am 40 miles north and haven't seen a thing... shows what being out of town does.
Anyway - from the stats given so far I cannot see enough to say there is any change in market share. But Nielsen tend to do their own ratings and these I would follow over and above our own experiences. (Follow the trend, not the absolutes).