The new rules will be effective 6 months after they are published in the Federal Register, but will not become mandatory until 18 months later. Until then, compliance with the old Standards will be permitted.
In short, start designing with the new requirements.
UPDATE: DOJ has issued final rules that revise (1) its Americans with Disabilities Act (ADA) title II regulation covering nondiscrimination on the basis of disability in state and local government services; and (2) the regulation that implements title III of the ADA relating to nondiscrimination on the basis of disability by public accommodations and in commercial facilities. DOJ has issued these final rules to have accessibility standards that are consistent with the minimum guidelines and requirements issued by the Access Board. Both rules take effect March 15, 2011. For more information read the factsheets that describe the major changes in the rules revising title II and title III. For additional information about the updated design standards read Adoption of the 2010 Standards for Accessible Design.