Learned yesterday that SIP consists of about 45 RFC's. Hence, why inter-operability is an issue, since it is still SIP, just depends what RFC's are supported. SIP Connect is a Specification, not a standard. Which means it is more like a guideline for getting the RFC's to work in an order than allows for a smooth packet path instead of packet-to-TDM-to-packet. (BTW, latency can increase depending on the number of conversions that the packet has to make).
What is a SIP trunk? Basically a PRI replacement that allows for IP from the PBX to the ITSP.
In the SIP Connect session it was suggested that Hosted VOIP can't scale. I think that truly depends on what scale you are talking about. As I have stated before, getting to 25k lines is HUGE --- and very few CLECs have hit that number (XO, the New Paetec, Cbeyond). Very few VoIP Provider will hit it. While Packet8 and delta3 probably have, it cost them millions over many years to achieve. So what scale? Sub-10K lines should work fine in a well designed architecture with redundancy.
In this same session, they mention that PRI Replacement is not a scalable opportunity either. The panelists from Neustar, L3, BandTel, & Cbeyond suggested that you need a Unique Value Proposition! Wow! Marketing 101.
BTW, the reason that L3 dropped the 3Tone VOIP service was due to research that suggested that most end users wanted a 25% savings to make the switch. "It was a losing proposition," according to Cary Tengler, Director of Partner Programs at Level(3).
One item stressed was that you can't do it all yourself. Strategic Partnerships that emphasize the strengths of all partners drives success.