In most cases, when the preview for a page is generated through our regular crawl, we will now render a snapshot of any Flash components on the page. This will replace the "puzzle piece" icon that previously appeared to indicate Flash components, and should improve the accuracy of the previews.
However, for pages that are fetched on demand by the "Google Web Preview" user-agent, we will generate a preview without Flash in order to minimize latency. In these cases the preview will appear as if the page were visited by someone using a browser without Flash enabled, and "Install Flash" messages may appear in the preview, depending on how your website handles users without Flash.
In this piece, Google goes on to give guidelines to improve the rendering of Flash in Instant Previews.
Sounds to me like SWFobject (open source) is still the way to go. I've been building sites with it for many years and it is the sweetest approach to getting visible content in front of a non-Flash user agent.