FYI, we (Jonathan and I) just came from a presentation on Terracotta. I think we have a pretty good idea now how we can support Terracotta best (just a couple of lines of code really) and we're convinced that this Wicket/Terracotta combi will probably be the way to go for most medium to large sized production environments with Wicket. At least it is the area where I would like to spend some more energy on when it comes to optimizing Wicket for scalability purposes.
I'll give a heads up on this in a new thread when we make progress on it. Eelco