In this post, we attempt to build a free, secure, SIP based communication system to provide encrypted voice and video communication, buddy lists, instant messaging, presence and remote desktop sharing/control on a self-hosted system.
I think they mean "in the context of a few private users using skype to communicate with each other", such as a startup or maybe even a tech-happy family.
To go big, we've horizontal scaling mechanisms using subscriber partitioning by load-balancing SIP and provisioning requests over multiple pairs of such servers (usually placed in blade-center servers).
The key here is to keep as much CPU heavy things like media relaying end-to-end where possible, because the signaling part is pretty light-weight in SIP. To scale out and keep reliability up while keeping complexity low, we have a shared-nothing approach wherever possible. Works well for us.