From Ned in a reply to this article: "The Senate was in "pro-forma" session when President Obama made the appointments. The Constitution empowers the Congress to make its own rules, including when it is in session. Historically other Presidents, including GWB, respected the Senate's "pro forma" sessions as valid and didn't try to make appointments during those times.
How often other Presidents made recess appointments, or how often appointments were filibustered or blocked in some manner, wasn't part of how all nine Justices looked at this case."