Steve Jobs has openly criticized Adobe Flash as being slow, buggy, insecure and crash prone… so it seems strange that the recent 10.6.4 update to Snow Leopard did not bother to include the latest version of Flash that actually addresses many of thoese concerns.
The version of Flash in 10.6.4 is version 10.0.45.2, while the latest version is 10.1.53.64, which not only patches numerous security vulnerabilities but implements support for hardware flash decoding under OS X… commonly cited as the number one reason why Flash works better on Windows than on a Mac. (Edit: As commenters below helpfully inform me, no, it jolly well doesn’t. Hardware decoding is coming in a future version of Flash for Mac.)
A conspiracist might think that Apple doesn’t want Adobe to fix Flash: they just want it to die. My guess, though, is that it takes so long to thoroughly test a software update that last week’s Flash update was simply too late to be bundled in.
If you’re concerned, just download and install the latest version of Flash yourself: it’s quite the improvement.