On Monday, July 13th the C++ standards committee voted "Concepts" out of consideration for C++0x.
First, skepticism regarding the feasibility and usefulness of concepts intensified the antipathy towards this proposal. Some people expressed concerns about compile-time and runtime overhead. Second, the creators of the Concepts proposal tried desperately to improve and patch Concepts. The last nail in the coffin was Bjarne Stroustrup's paper "Simplifying the Use of Concepts" from June. It's a masterpiece in terms of presenting Concepts but it also scared folks. The general sense was that concepts were broken, the committee was not sure what the correct direction was to fix them, and it would probably take several more years to come up with a reasonable fix that would achieve consensus. Considering that Concepts were originally designed to simplify C++, a critical mass of committee members agreed in July 2009 that it was time to bid Concepts goodbye.
For more on the meeting see "The View (or trip report) from the July 2009 C++ Standard Meeting" part 1 and part 2
Bjarne Stroustrup on The C++0x "Remove Concepts" Decision.