It never happened. I have IBM stickers covering up all the Lenovo markings on my Thinkpad
IBM lost control of the PC market yonks ago. Their strategy was/is to build high quality stuff (for the most part), give you loads of tech support, but on the other hand they charge you through your nose for it. They ended up in a catch 22 situation whereby if they tried to cut corners, there would be nothing to distinguish them from the competition, and it would damage their reputation with high end stuff where they made all their money (servers, mainframes etc)
The main issue was that the PC market is one of those consumerist disposable ones where really high end stuff doesn't have much more longevity than lower end ones. IBM nearly went bankrupt over attempts to reconquer the PC market and some other bad moves, and they had to undergo a huge restructuring whereby they focused more on providing 'services' than actually making things.
In 2005, they finally decided that the PC market wasn't profitable for them any more, and they sold their PC division to the Chinese company 'Legend', who took over IBM's manufacturing operations in China, and earned the right to produce IBM's Thinkpads and Thinkstations. For the first 2 years or so, they even earned the right to use the IBM logo on the Thinkpads. Legend became Lenovo, supposedly a combination of 'Legend' and 'Noveau'.
Of course, it wasn't the first time IBM had done such a thing. About two years previous, they sold their Hard Drive section to Hitachi after the infamous "Death Star" fiasco. And of course, in 1991 they divested printers, keyboards and typewriters to Lexmark. It's a pretty common thing amongst huge corporations where they overstrech themselves providing for every conceivable market, and they have to shed the dead weight every now and then as a consequence.