Apple has released Mac OS X 10.5.4, the fourth update to Leopard since it was released last October.
The new version contains the usual mix of bug fixes and security updates, with iCal getting the most attention. iCal won't delete events without telling you as a result of the latest update, for example, and Apple said the update "improves overall iCal reliability." Airport and Spaces & Expose also received some updates.
There are also a couple of security-related fixes for Safari and other issues that my colleague Rob Vamosi will cover in more detail. Mac OS X 10.5.4 should pop up through Software Update any moment now, or you can force the issue by visiting Apple's site.
For what it's worth, 10.5.4 did not appear in the list of available updates when I ran Software Update on my Open Computer from Psystar, as expected. As of about 2 p.m. PDT, Psystar hadn't posted a version on its site, although it might take the company some time to get the Open Computer version of the update up and running.