WordPress officially announced and recognized the bug with pingbacks and curl with the WordPress 2.9 release and are hard at work to release 2.9.1 which will address these issues. If you have already upgraded to WordPress 2.9 the WordPress team could use some beta testers willing to download the beta 1 version of 2.9.1 and put it through its paces.
Here is the official word from WordPress.org blog:
Unfortunately, the recent 2.9 release triggered a bug in certain versions of PHPâ€™s curl extension. 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. The easiest way to test Beta 1 is to install the WordPress Beta Tester plugin, elect to get on the point release development track, and then perform an automatic upgrade via the Tools->Upgrade menu. You can also download the Beta 1 package and install manually. Fourteen tickets have been fixed in 2.9.1 Beta 1. Since the curl problem and a couple of other problems are dependent on specific hosting configurations, any and all testing help is greatly appreciated.