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Apparently this is due to a bug in certain versions of PHP's curl extension. Right now this maintenance release is in beta until they determine for sure that it resolves the issues correctly without creating further ones.
From the announcement:
With these versions of curl, scheduled posts and pingbacks are not processed correctly. To fix this problem as well as a handful of other, lesser issues, we are quickly releasing 2.9.1, the first maintenance release of the 2.9 line. Help us get 2.9.1 ready to go by testing 2.9.1 Beta 1.
You can grab the release from the release announcement page.