Nice one. But, it doesn't mention some of the most significant problems, which imho are caused by firewalling. SIP does all sorts of clever things to try to get around NAT/firewalls (notably the rather clever trick of using STUN), but nevertheless, unless you have a public proxy server which is accessible by both parties to the call (and which is willing to handle both the low-bandwith call setup AND the high-bandwidth call data), it's very hard to be reliable.
Trying to get SIP to work for an inbound call to someone behind a corporate firewall is nearly impossible. The public proxy server approach works, but this is too expensive for an organisation like Ekiga to handle (Ekiga can easily provide the free, low-bandwidth address-book service, but not proxy the calls themselves).
It pains me to say this, but Skype get it right, and nobody else does.