Tomoli,
Do not waste time and energy researching solutions that are not meant to be one in the first place. Using SIP proxies/registrars adds additional load and point of failure.
My sense is that the people here that looking for integration with Cisco VoIP do so because they have already existing infrastructure and can't just get rid of it at once. With my respect to the third party VoIP
PBX developers, being able to interconnect OCS with VoIP provider via middleware does not mean this is the ONLY solution.
Read this forum from about the beginning of 2007 and come back for discussion.
Drago