First thoughts on Adobe's Apollo
For an alpha release, Adobe's Flex-HTML-PDF desktop runtime is fairly funky.
There are gotchas (check the ApolloCoders mailing list) but in general it took an afternoon to go from writing web-based Flex (or Flash, or (D)HTML, if you prefer) applications to building installable desktop versions of the same - using both the free SDK and the plugin to Flex Builder / Eclipse.