I don't know about the rest of the world, but that's nothing new in Europe, the first time I read about this was back in 2005... And I remember Gartner or GfK also publishing on it later on. Here's the original publication, for reference (lucky I archive everything):
Tuesday, 2nd August 2005
NOTEBOOKS OVERTAKE DESKTOPS IN MAY 2005
Notebook sales have overtaken desktop sales in Europe’s top eight economies in May 2005 with 51.3% of the market, according to figures published by Context, with the UK the latest to see notebooks eclipse desktops. Italy still leads the way with 62.2% of notebooks.
Context’s SalesWatch research, tracking sales of business and consumer technology products through indirect channels of distribution in Europe, revealed that as notebook sales increased by 18.4% compared to a year ago, desktop sales fell by 12.1% over the same period, raising the proportion of notebooks from 43.9% to 51.3% in May 2005.
The UK joined the group of notebook-dominated markets with 55.4%, compared to 38.5% a year ago, while Italy is still ahead with 62.2%. The Netherlands and Sweden could follow soon with less than 55% of desktops in May 2005, but France remains behind as desktops still accounted for 61.2% of total PC sales.
Bill Gates, because you can't buy a PC laptop without Windows, although tied sales are illegal... Gotta be good news for somebody!