Thanks guys. So it's actually a new build, although not a new version.. presumably it's just that the flash plugin has been updated. Or actually, no. I checked, the flash plugin is its own package: macromedia-flashplayer. apt-cache says it's version 1.20-1. Looking at the plugin itself it turns out that this is still flash 7:

$ grep About /usr/lib/browser/plugins/libflashplayer.so
About Macromedia Flash Player 7...

So hopefully we can expect more improvements to flash in later upgrades (version 9 is supposed to be faster, isn't it?).