A recent change has been communicated for all current Windows SharePoint Services 3.0 B2 (Beta 2) customers. The change will require customers who wish to upgrade their B2 environments to first apply B2 Technical Refresh prior to upgrading to RTM, Windows SharePoint Services will not support B2 to RTM direct upgrades.
Previously announced upgrade paths will remain unchanged at this time:
B2 Technical Refresh remains planned as a release patch, upgrading to RTM will require the modification of a Registry key, uninstalling B2 Technical Refresh, and then installing RTM. This will be documented in both B2TR and RTM.
I'll post additional information on this change as it is made available.