Seam is built to take advantage of the features of the underlying Java EE implementation. For example, the conversation context data is actually kept in the HttpSession when server-side conversations are used.
So no, there should be no loss of data when a server fails ;-)
I was just putting together a small clustered environment to test this first hand. I think I'll go ahead and prove it to myself anyways. :)
Nice, let us know if you have problems.