They expire one year from activation (or on-site purchase), not first use. If you activate (or purchase on-site) on 4/10/2011, they will expire at midnight on 4/10/2012.
What Michael is saying is that if you buy it at WDW, it's activated right then. If you buy in via the internet, it's activated the first time you use it. But if you pay for a renewal, it automatically adds a year to the end of your pass.
Regardless of purchase method (phone, online or onsite), you have to go to guest services or a theme park ticket window at WDW to get your actual AP. If you did this on 4/10/2011, your pass would expire on 4/10/2012. They are good for 1 year from the date you actually get them.