Over at the Rhinoceros - Next Release Web page, Robert McNeel describes how the process in updating the software:
* Work-in-Progress releases (aka "alpha" release) lasts 3 - 6 months.
* Beta versions are ready for production, but do not have the final user interface; another 3 - 6 months.
* Final release is roughly 12 months after the first WIP.
Other items of interest:
* They are tracking 2,000 wish list items.
* Rhino 4 will run on Windows 2000 and XP only.
* New features under development include: Mesh tools, Solid editing tools, Advanced display, Sheet layout, SmartTrack, Universal Deformation Technology, Creation history, PDF, AI 8+, DWG 2005, and DGN import.