According to DAllen in this thread, yes, as long as the Scala classes are not final.
I just gave Scala a quick test and since it supports annotations and all, if you compile the classes, you can use them in Seam (and therefore WB too). One thing to look out for is that you cannot create final classes since Seam needs to proxy them.
In addition to the distributed tx Seam example I'm working on, somebody should submit a Scala-ized version of the booking example...
Scala is definitely HOT and gaining traction (it's used in the codebase for twitter.com now)!
To remain competitive and bleeding-edge as it always has, Seam should not ignore Scala.