I'm sure it's just a coincidence, but Apple seems to be developing a habit of releasing Mac OS X updates while I'm away. Last week's arrival of 10.5.5 is a case in point - the day after I go overseas, a major update appears.

I sometimes think Apple's doing me a favour, as it reduces the temptation to leap in and update my computers as quickly as possible. Instead, there's an enforced delay which provides an opportunity for others to detect any significant issues that may be lurking.

As it happens, there don't seem to be any such issues that would be a concern for me, and most - though not all - of the gripes seem to be specific rather than systematic. You'll always get a few people claiming that an update should be avoided because it wrecked their computer, but it seems to me that the most common cause is a system that's not quite right in the first place.

This position is based on the fact that running the combo updater (which updates all the files changed since the last major release of the operating system, not just those new to the point release) often cures update issues. Sure, that leaves open the question of how the corruption occurred and why an update can't check the integrity of existing system components.

Anyway, what does 10.5.5 deliver? Nothing in the way of new features, so there's no major cause for excitement.

Firstly, there's a swag of security fixes that are, where applicable, also available for 10.4.11 as Security Update 2008-006.

As far as 10.5 is concerned, 23 security fixes are delivered by the update, including new versions of four pieces of open-source software included in the operating system. Those new versions typically fix multiple vulnerabilities, so these numbers aren't an excuse to trumpet the greater security of open source software vs proprietary code.

Once again, the update patches flaws that could be exploited by maliciously crafted media files (in this case TIFF and JPEG images), potentially leading to the execution of arbitrary code.

There's also what should be the final part of a fix for the 'Kaminski' DNS cache poisoning issue, though it isn't clear which (if any) programs actually use the libresolve component that's now been updated. The BIND DNS software that runs on Mac OS X Server was addressed by a previous update.

What else was fixed? While Apple is notorious for being vague about the content of software updates, it published a reasonably detailed list for 10.5.5.

Changes that caught my eye include improved Spotlight indexing performance, extensive graphics enhancements, and improved reliability of MobileMe syncing (so presumably part of the problem was the fault of Mac OS X rather than MobileMe itself).

There are also potentially significant changes to Address Book, iCal, Mail and Time Machine, none of which I use on a regular basis, as well as fixes for various stability issues with the MacBook Air. Those of you who wish you had an Air to give you stability problems can join the queue!

One warning I have seen about the 10.5.5 update is that it is another case where more than one restart may be needed and that it is important to give the computer plenty of time to sort itself out after restarting. It's not the first time that's been the case, but if you haven't seen your computer grind away after a post-update restart, don't panic if it happens when you apply this update.

Apart from 10.5.5 and the security update for 10.4, Apple's also recently released updates for RAW compatibility for Aperture 2 and iPhoto 08, new server admin tools, an Apple Remote Desktop update (client and admin), Final Cut Server Update 1.1.1, Pro Tools Update 2008-003, and a new iPhone Configuration Utility for Mac OS X.

QuickTime 7.5.5, Front Row 2.1.6, iTunes 8, and a new collection of HP printer drivers all arrived earlier in September.

So if you haven't run Software Update recently, now is probably the time.