I'd use an alternate browser for the ad blocking plugins available.. I'd give up Flash support in a heartbeat for an ad/script blocking plugin for the Playbook native browser, and I'm talking a selective blocker like AdBlock Plus on Firefox, not just the capability to wholesale turn off scripts and flash like in the current browser..
Script-heavy sites noticeably bog down the Playbook browser (get lots of checkerboarding).. Ad blocking has made my surfing 1000% faster and these days, I'm finding the PlayBook browser very sluggish in comparison to my desktop/laptop.. With the old leaked Android player, you could use Dolphin (and I seem to recall that it did support Flash), but most importantly, if you followed HaTax's instructions and installed Android Marketplace, you could also install the ad blocking plugin, and it worked great..
I'm hoping that since the NDK is available, someone will try their hand at porting Firefox or Chrome over..
This work on my PB on the stock browser. You can't see the Movie but it is there and playable. Doubletap (normaly on doubletap the movie goes into fullscreen) on the area where the movie is and the Video goes into fullscreen and you can watch it.