OS X Update Issues
It seems that since Leopard 10.5.1 not only I, but many OS X users, have encountered updates problems when automatically updating from 10.5.1 to Mac OS X 10.5.2, 10.5.2 to 10.5.3, 10.5.3 to 10.5.4, etc., etc. The problem experienced by many during the last three updates is that the OS is hang at reboot, where the apple grey screen will appear with the little spinning circle and not progress further than that point.
From what I can gather, reading feedback in a number of Mac related forums, this appears to be a problem with the updating process, and most likely has to do with a permissions error of some sort not being corrected before the update, and subsequently affecting the update.
The good news is that for most users just waiting it out and allowing the problem to correct itself has been the answer. In many cases, users have just not waited long enough for boot problems to self-correct. However, if the problem does not correct itself after several hours, then it is time to fix the drives permissions. For details see my tips entry on how to do that.
Experts also agree that instead of updating OS X via the automatic software update process to do it the manual way by first fixing the permissions and than using the updates that are available manually from the Apple web site. The installer in that case performs better binding than the automatic process. I have added a tips entry on the steps required to perform the safer manual OS update.