To have WebRTC capabilities on IE, users could install Chrome Frame (for IE6+). It is a valid option, but you should know that it is no longer being actively supported by Google.
As far as I know, there is no automatic switching library. However, on your server, you could look at the HTTP requests and find out the client's browser. From there, you could dynamically load either the WebRTC or Flash library depending on the browser's support for WebRTC. Currently, the OpenTok WebRTC library supports:
- Chrome 23+
- Firefox 22+
Unfortunately, OpenTok does not interoperate between Flash and WebRTC clients. WebRTC clients can operate with other WebRTC clients (mobile, web, etc.), just like Flash can only operate with other Flash clients. For example, an iOS client would have to use the WebRTC SDK and the Chrome/Firefox web app would have to include the WebRTC Javascript library.