Since Metro IE is plug-in-free by default, Flashworks without any additional setup required. (If you want to disable Flash in the desktop version, though, you'll find that option in the add-on manager.) And in case you're wondering about Flash's diminished role on mobile devices, well, we're told it'll work with both x86 and ARM devices.
One notable plugin hangup is the fact that Flash only works in the browser, and can't be embedded into a regular webOS app, PDK or no -- though we have to assume this is something that's in the works.