If engines can't read Flash then what about putting your keyword content inside noscript tags? You could also use HTML for your links and display some stuff in Flash. I think it's do-able to get a flash site to rank high.
What about using a Flash CMS that promises SEO?
The reason I question using Flash for IM is because I think Flash provides a more professional looking element, which might lead to better conversions(?). I would be more inclined to use Flash on a one paged sales page or on a domain where my keyword is my domain name.
I think if one where to use Flash properly, then it could have it's place in the IM world.
Also, has anyone tried AdSense in a flash site?