The FCC has extended the 'Do-Not-Call' Registry so that numbers will not be automatically be removed. When it was first set up in 2003, numbers were to be removed after 5 years. I, personally, have had the experience of numbers being removed after a couple of years, so I still recommend checking from time to time to be sure your numbers are still on there.
For more information:
http://blog.washingtonpost.com/small-business/2008/06/fcc_extends_donotcall_registry.html
http://www.ftc.gov/donotcall