X-bit Labs reports ATI is no longer developing Sideport. The technology was introduced together with the Radeon HD 4870 X2 graphics card, it was supposed to boost bandwidth between GPUs but it was never enabled. The new Radeon HD 5800 series don't support it, and it appears the technology may never see the light of day.
It turns out that Sideport will never seen the light of the day, perhaps, due to complexities of synchronizing data transfers between different interconnections. Maybe, when the multi-GPU technologies used by consumer-oriented solutions change radically and extremely high-bandwidth will be required for interconnection between the chips, Sideport will be back. However, not with the ATI Radeon HD 5800/5900-series.