You may have already seen the Antimalware blog posting on this and you can read the full story here.
http://blogs.technet.com/antimalware...01/444384.aspx

Over the next two weeks the Antimalware team will be testing a new definition update process and, as part of the test, they will be delivering signatures daily Monday through Friday through August 15. After the 15th updates will return to the current schedule of twice weekly.