Pushing back the Fedora 18 schedule was agreed upon due to a number of outstanding blocker blugs and insufficient test results for the current base, installation, and desktop tests. At the moment there's just shy of two dozen Fedora 18 blocker bugs as shown by Fedora QA.
As said on the Fedora test-announce list, "As a result, ALL MAJOR MILESTONES, and their dependent tasks, will be pushed out by one week."
As a result of the slip, the Fedora 18 Alpha is now scheduled for 4 September, the Fedora 18 Beta for 9 October, and the Fedora 18 final release for the 13th of November.
There's a tradition to delays within the Fedora camp so today's setback of one week isn't entirely surprising nor will it be surprising if other delays crop up by the time of the planned Fedora 18 "Spherical Cow" release in mid-November. In fact, it will be surprising if there are not anymore F18 delays.
At least though Fedora Linux puts its standards higher than some other distributions than wanting to just ship on a commemorative date (e.g. 10.10.10 for Ubuntu 10.10) and that they have real, committed upstream innovations going on. For the upcoming work in this Red Hat sponsored Linux distribution, see the best features of Fedora 18 -- there were also some feature freeze exceptions earlier this week.