The most significant thing about what's changed in OS X 10.5.4 may be what hasn't changed with Monday's release of the OS X update. The ARDAgent security hole--discovered just a few weeks ago--remains unplugged. This actually isn't all that surprising--if the chatter around the Internet is to be believed, Apple has been working on the 10.5.4 update for quite a while, predating the discovery of the ARDAgent issue. System updates are complex things with many inter-related parts, and adding in a fix for the ARDAgent issue would have required more testing, and perhaps delayed the release of 10.5.4.
So what will happen with the ARDAGent hole? Only Apple knows, of course, but I think we'll see a standalone security update released in the near future to address that issue (as well as any other security issues that haven't been patched with the 10.5.4 release).
As for new things introduced in OS X 10.5.4, I dug around a bit in the bom files, as I did with May's 10.5.3 release. This latest update is much smaller than 10.5.3, and I didn't find anything nearly as interesting as I did last month, other than the non-fix for ARDAgent. There are a slew of extension updates, though it's tough to figure out exactly what changes those may entail.
There were a number of updates to PDF-related utilities, including the Mail PDF, Save as PDF-X, and Save PDF to Web Receipts Folder workflows, as well as changes to many PDF-related Automator actions. Non-English language files in many programs were updated, and there was some sort of change to Mail's preferences, though I couldn't see any obvious change when compared with Mail on a 10.5.3 machine. iChat received updates to the balloons, boxes, and compact styles, as well as some (also not obvious) changes in its preferences.
The Apple-provided dictionary in Dictionary was also updated, adding (at a minimum) the definition of MobileMe, Apple's soon-to-launch online service:
An Internet service from Apple Inc. for Macintosh computers, iPhone, iPod touch, and PCs. A MobileMe subscription provides push email, push contacts, and push calendar to keep your data automatically up-to-date on all your devices.
Given the size of the updated file, I expect there are more new definitions, but MobileMe was the only one I could think to check.
There are also changes on the Unix side of OS X. Some Perl and Ruby related bits were updated, as were snmp-related Unix programs, and various other Unix components, such as the pasteboard server.
I've been running 10.5.4 on two machines now for a few hours, and haven't experienced any issues--though as with any update, I strongly recommend having a current backup before proceeding.
This story, "Inside the OS X 10.5.4 Update" was originally published by Macworld.