Firecore's aTV Flash is a US$50.00 replacement for the stock Apple TV software that enhances the settop box's capabilities to include web browsing, access to music on Last.fm, external USB drive support, and playback of more video formats. Ironically (considering its name), Flash code itself is NOT supported.
That may change very soon, as Firecore announced the immiment arrival of a new aTV flash, to accompany the next generation of Apple TV boxes, that may well support HTML-5 streaming and possibly (emphasis on "possibly") Flash support. If the latter happens, it would likely be through a server-side conversion to an iOS-compatible format a la the Skyfire browser.
All should become clear in the coming weeks, when Firecore releases a public beta of the new aTV Flash.