The version 1.0 of Croquet has been released today; no beta anymore. This is for sure a great step for this project and I am excited to see to which place this is going to lead this project now!
BTW, I've found another 3d-project today on my radar: Uni-Verse. Founded by the European Union, developed by academic institutes, seems the purpose of this project more in the field of 3d-collaboration. It's, like Croquet, also Opensource and the demonstration movie on their website looks very interesting. It has also interfaces to Maya and Blender - no wonder since the Blender foundation is involved in the development of Uni-Verse.