While it might seem a no-brainer to allow CPAs to practice in other states without jumping through licensing hoops, the issue as not as black and white as you might think. When you remember that each state gets to set the rules for how a CPA gets licensed, then the mobility issues start to make sense. If State A requires you to practice as a public accountant for two years before you can be licensed, and State B says you can be licensed after one year of work experience, and that experience doesn't even have to be in a public accounting firm, how does State A know that the State B accountant has the chops to practice in State A?
Remember that it's one thing to have a CPA certificate, but another thing altogether to be able to hold yourself out as a licensed CPA. Some states don't require work experience at all to be licensed, other states won't even let you sit for the entire exam until you strap on a green eyeshade and log in some time in an office. Some states require an ethics exam, others do not. If your state requires an ethics exam, how can it justify allowing someone from a non-ethics exam state to practice there? See - the waters are pretty muddy when it comes to mobilization issues.
In the end, each state still has the right to decide who can hold himself or herself out as a CPA, but hope springs eternal and with the help of the Internet, the influx in interstate commerce is causing states to get on the mobility bandwagon. The new NASBA/AICPA CPA mobility Web site will help you figure out if your license will be taken seriously in other states. Unfortunately the site falls short of telling you what to do if mobility is not allowed in a particular state. "Contact the state board to determine the availability of other practice alternatives" isn't a solution - easy-to-maneuver links to the appropriate alternatives and state boards would be useful.