I'll have to defend Apple on this one....
Not only is this OLD version of Flash vulnerable, it happens to perform worse too.
There's absolutely no excuse for an upgrade replacing software with versions that have security issues.
It's difficult to run something as complicated as an operating system through quality assurance - you have to pick a set of files (the OS build, plus other tools and 3rd party apps) from a point in time, and test those, and if the tests pan out you ship those.
Clearly an application change, even a security update, that occurs after that point in time is not included. (If the "point in time" snapshot doesn't include fixes that were published before the "point in time", then criticism is due.)
Look at it another way - what if tomorrow (2009/09/04) Adobe patches a critical security flaw in Flash.... What if you're running 10.5.999, and you patch your system from Adobe.... What if you upgrade to 10.6 next week.... Do you blame Apple for exposing you to a flaw that you've fixed on your system, even though that fix wasn't known when the DVDs were pressed?
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The only potential for criticizing Apple is whether running software update immediately after upgrading the OS fixes the problem.
With the "continual improvement" philosophy in Windows Update, one expects to run Windows Update immediately after an install, and you expect that problems like this will be taken care of. (People who install Windows 7 on the October 22 launch will see a bunch of updates waiting....)
If Apple users have to wait for Apple to test and release 10.6.1 to fix a known vulnerability - well, that's not so good if they have to wait a long time to fix a problem....