Of the wrestlers on that list, the only one to have quit was King Booker. Dave Meltzer reports that Booker quit WWE when he was told that he was to be suspended for 60 days because he was believed to have failed a Wellness Program test before, hence the 60 days suspension instead of 30.
According to Meltzer, WWE is still trying to keep Booker, however they're going on under the impression he's done. The two sides had been having troubles recently, with a major setback being John Laurinaitis' call to go with Florida for a developmental territory - where WWE would start from scratch - instead of going with Booker's already established PWA, which WWE had been loaning wrestlers to. After they made the call to stop that, Booker went to his friends at TNA and booked them for his shows instead, which didn't sit so well with anyone backstage, but they couldn't say much having pulled WWE talent from PWA.
Booker is pretty much set financially and has wanted out of wrestling for a while. At age 42, he's accomplished a lot, and thanks to his hip hop store doing well in Houston, and his two wrestling schools up and running, there's really no need for him to stick around.
The other major star to have quit was Ric Flair. According to the Wrestling Observer newsletter, Ric Flair gave his notice to WWE on or around 8/20. As you may have guessed, Flair was upset because he's being underutilized. Several months ago, Steve Austin came up with a storyline for Ric Flair where Flair would lose several matches and then announce he would quit if he lost another match. That would start a winning streak for Flair that would culminate at next year's WrestleMania. Austin pitched the plan to Vince & and Stephanie McMahon and they loved it. However, writer Gerwitz was against the idea, feeling that for it to work Flair would have to beat top talent on the roster, and it would make them look weaker losing to the older Flair. Flair was obviously on board when he heard about it. Flair was not promised that he'd win the title, but was told he may get the gold if the storyline caught on.
At this time, he is still under contract and still getting paid by WWE. Most people within WWE expect Flair to return to the company. Flair was on local sports talk show Prime Time with The Packman on 9/9/2007 on WFNZ in Charlotte. He said he will be opening a new business in Charlotte this week called Ric Flair Finance. Flair talked a little bit of football before the host asked him if he really quit WWE. Flair kinda laughed at it and said "No, it's not true." and asked what WWE would do without him. He didn't seem too sure of himself but he said he would talk more about everything next week on the same show. Flair and WWE continue to talk after a creative disagreement. He remains under contract with the company and it is expected he'll be back after things smooth over and a deal is reached.