Wikimedia Foundation developer liaison Tim Starling quit Wikipedia cold turkey last Tuesday. On the heels of this, he was banned by Jimbo Wales as well.
Indicating that his work in his Ph.D. program was suffering, and frustrated by constant demands for developer attention to matters he didn't want to be working on, Starling said he would leave Wikipedia entirely. However, he did not swear never to come back, and Wales was quite sure he would return eventually. (The "ban" was imposed so Starling would "get things straightened out" with his obligations outside of Wikipedia. Wales added, "Now, if only someone would ban me.")
Recently, of course, Starling and other developers spent a difficult few weeks struggling to restore Wikipedia performance to acceptable levels. At one point, he commented, "I discovered over Christmas that I could work 40 hours per week on Wikipedia system administration, and I wouldn't be able to keep up with the demand".