Just to throw an observation in here. I watch a 4 year-old site that is largely Flash based. The screenshot shown in SERPs is horrible - a huge white space (where the Flash application would be) a very basic header graphic, an advertising banner and lower down, some text in a very old-skool/fluid width format and not very attractive design.
But, because of the Flash content, the site holds the attention and it has gone from strength to strength through both Panda and Penguin, pretty much now one of the Top 3 dominant sites in it's relatively competitive sector. Graphically and design-wise, it's got "not updated since 1994" written all over it so if Google looks at the screenshot, I can't see how it would have anything more than a very small impact in the grand scheme of things.