As per the announcement, XenForo will be ready at the end of the summer, maybe, therefore i think if there is a testing face, it'll be around that time. I'd love to be in a beta testing team if there will be one.
It is currently in it's first alpha build, and being worked on as we speak. We're privileged here to try and play with it, use it and give feedback and report bugs. It's way to early to talk numbers, dates, roadmaps, and alike.