For new construction and alterations beginning on or after March 15, 2012, a covered public or private entity must comply with the 2010 Standards for new construction and alterations.  

 If construction or alterations start before March 15, 2012, you have a choice of following the 1991 or 2010 Standards.  If construction or alteration might not start before March 15, 2012 (e.g., for a new project that is in the design stage), it will be safest to use the 2010 Standards for that project.

You may want to consider the type of building and the types of alterations you contemplate before the compliance date.  For example,  an auditorium or theater with tiered seating have to follow less stringent requirements under the 2010 Standards (in most respects) than under the 1991 Standards.  If you alter a single-user toilet room, in many cases the 2010 Standards would require increased floor space compared to the 1991 Standards.   

 During the transition, you can't choose to follow one standard for part of a building and another standard for another part.  In other words, all alterations to a building during the transition (from March 15, 2011, to March 14, 2012) must follow the one standard you choose.

What happens in a State that does not use the ADA as their accessibility guidelines?

One purpose of the 2010 Standards was to harmonize the federal requirements with state requirements. In about half the states, new construction and alterations already have to comply with a state code that is very similar to the 2010 Standards.  

If you are in a state that has adopted the 2003 or 2006 International Building Code (including the accessibility standards of the American National Standards Institute, A117.1-2003), then when you build to your state requirements, you will be following most of the federal requirements already.  But  you will also need to follow the additional requirements that DOJ has issued "beyond" 2004 ADAAG.

For example, in Texas, the Texas Department of Licensing and Regulation is reviewing the 2010 Standards to determine the next course of action.  They are speculations that they will adopt the 2010 Standards and make it the Texas Standards at the same time as the Federal Standards.  We will keep you posted as we find out.  Until such time, designers will have to design to the State guidelines and use the more strict interpretation if conflicting with the 2010 Standards. 

Both the 1991 and 2010 Standards generally require that when existing elements and spaces of a facility are altered, the alterations must comply with new construction requirements.  If there is an existing facility that meets the 1991 Guideline requirements, then The 2010 Regulations provide a "safe harbor" for those elements.  Those elements do not have to be modified in order to meet the 2010 Standards, just for barrier removal purposes. You should document your compliance as to those elements, before March 15, 2012.

An element that does not comply with the alterations provisions of the Standards for that element is considered a "barrier."  For example, a typical round knob on a door that should be accessible would be a barrier because it requires tight grasping, pinching, or twisting of the wrist to operate, contrary to the provisions of the Standards. Changes undertaken as part of a barrier-removal effort should comply with the Standards.  

Note that this safe harbor does not apply to elements for which there are no standards in the 1991 Standards, such as residential facilities and dwelling unit, play areas, and swimming pools.  DOJ lists these in the 2010 Regulation at section 36.304(d)(2)(iii). If there is a building that does not meet the 1991 guidelines currently, or do not fall under the "safe harbor" provision, the DOJ recommends the development of an implementation plan for compliance with the barrier removal provisions, and an ongoing method of assessing compliance.  As a matter of good practice, and to the extent your resources allow, the plan should include a list of significant access barriers (created after a survey of the building), a schedule for removing them, and a description of the methods used to identify and prioritize barriers.  

 Remember that you have only until March 14, 2012, to do barrier removal under the 1991 Standards.  So you should start now to assess barriers under those Standards, and remove any that are readily achievable to remove.