See http://converter.tipodean.com/unity3d/index.html for details of Tipodean’s OpenSim to Collada for Unity3D conversion service.
The Collada translation of the open source OpenVCE assets suitable for import into Unity3D was kindly provided to Austin Tate by Chris Collins of Tipodean Technologies for experiments and an early trial version is available at http://www.aiai.ed.ac.uk/~ai/unity/iroom/collada/
For other experiments with I-Rooms in Unity3D see http://www.aiai.ed.ac.uk/~ai/unity/
Note that the multi-user severs (using Smart Fox Pro) at Edinburgh are required for some of the demonstrations and are not always left running.
Update – September 2015 – Tipodean OpenSim OAR to Unity Converter
As at September 2015 http://tipodean.com no longer seems to be available, but the converter web site and previous examples, including multi-user OpenVCE region demonstrations, with and without VOIP, continue to run at http://converter.tipodean.com/unity3d/index.html.