For another perspective on the issue, we asked BES guru and BlackBerry Forums staple, Jibi, what he thought:
To be honest, I’m not sure why the 8×20 devices wouldn’t support SIP protocols. Perhaps it really is carrier-driven (and why wouldn’t it be). The devices were developed with UMA/GAN in mind (read: T-Mobile US), so I’m not sure if SIP became an afterthought or if it will be supported in future OS revisions (I can’t imagine that it’s not supported by the hardware). I’m not really sure if anything is being developed in conjunction with MVS/Ascendent at this point, although I do know the “Enterprise Phone” feature will be standard inclusion for the OS 4.3.x devices and above (perhaps future versions of all OS releases).
Concerning the PBX integration and SIP exclusion, RIM obviously has some agendas, both from business and security angles, so I think you hit it right on the head when you quoted their wanting to do for voice what they’ve been doing for email. Perhaps that’s not the best way to go about it, but if the solution is standard billing for BES 5.0 and if BES is already an integral part of the messaging infrastructure, then their voice integration is either going to flop (due to already established unified communications integrations) or it will be very successful (kind of like people choosing Microsoft products within a Microsoft-based environment due to the nature of the integration of the Microsoft product families).