Indeed, I'd say that having a text version as well is handy. But just bare in mind, Google for example, as well as the page body, it mainly searches in the <title> but also in the meta description tag.
At least I presume it uses the description tag because that shows up in the search results if it's available.
So, yes, I'd say maintain a text version as well just for the purpose of SEO... But obviously presume people will enter the site that way, and recommend them to switch to the flash version so all your hard work doesn't go to waste.