Thanks, and probably will... I am usually not bleeding edge. If I was I would have been fired years ago.
However, I have instructed the staff and our student help to install 10.6.4.on test machines using a real account and SW package and try like hell to break it today.
Why?
I have been playing quite hard at home here with it and it is THAT good of an update not to at least see, and there are a few things like CS3 fixes that are a compelling enough reason.
Just reading through the Apple news digest of fixes etc. that came with this update, to be honest the update seems quite complete and is in your interest to apply it to all your machines (security point of view more than anything) so agree with you on trying it out on test machines and trying to break it

TODAY as you can see from the below list there are quite a few things that would be off interest to apply A.S.A.P.
You can subscribe at
APPLE LINK
Things fixed in this update includes
DesktopServices
Impact: A Finder operation may result in files or folders with
unexpected permissions
Flash Player plug-in
Impact: Multiple vulnerabilities in Adobe Flash Player plug-in
Description: Multiple issues exist in the Adobe Flash Player plug-
in, the most serious of which may lead to unauthorized cross-domain
requests. The issues are addressed by updating the Flash Player plug-
in to version 10.0.45.2
Folder Manager
Impact: Unmounting a maliciously crafted disk image or remote share
may lead to data loss
Help Viewer
Impact: Visiting a maliciously crafted website may lead to the
execution of JavaScript in the local domain
iChat
Impact: A remote user may upload files to arbitrary locations on the
filesystem of a user currently using AIM in iChat
ImageIO
Impact: Opening a maliciously crafted TIFF file may lead to an
unexpected application termination or arbitrary code execution
Impact: Viewing a maliciously crafted movie file may lead to an
unexpected application termination or arbitrary code execution
Kerberos
Impact: An unauthenticated remote user may cause an unexpected
termination of the KDC process, or arbitrary code execution
Impact: A remote user may cause an unexpected termination of the KDC
process, or arbitrary code execution
Impact: An unauthenticated remote user may cause an unexpected
termination of the KDC process
libcurl
Impact: Using libcurl to download files from a maliciously crafted
website may lead to an unexpected application termination or
arbitrary code execution
Network Authorization
Impact: A local user may obtain system privileges
Impact: Visiting a maliciously crafted website may lead to an
unexpected application termination or arbitrary code execution
Open Directory
Impact: A man-in-the-middle attacker may be able to impersonate a
network account server
Printer Setup
Impact: Network devices may disable printing in certain applications
Impact: A user with access to the printer may cause an unexpected
application termination or arbitrary code execution
Ruby
Impact: A remote attacker may gain access to accounts served by Ruby
WEBrick
SMB File Server
Impact: A remote user may obtain unauthorized access to arbitrary
files
SquirrelMail
Impact: Multiple vulnerabilities in SquirrelMail
Wiki Server
Impact: Viewing maliciously crafted Wiki content may result in a
cross-site scripting attack