Originally Posted by fastmhc
Maybe I don't? this is a quote for the wiki page for GPS navigation for phones
" Some phones with GPS capability work by assisted GPS (A-GPS) only, and do not function when out of range of their carrier's cell towers. "
Most of today's cell phone GPS chips can work either in standalone mode or Assited mode. A(ssited)-GPS helps to speed up your initial satellite lock by downloading satellite information from network. If you have no data connection, the chip will fall back to standalone mode. With A-GPS properly working, you can achive first lock within 10 seconds. Without it, you may have to wait for couple minutes for your first lock. After the initial lock these GPS chips work autonomously unless you use MS-Assited A-GPS mode. With the power of today's Smartphone, there is zero reason for using MS-Assisted mode unless you're using it in E911 situations.
It is ultimately the navigation software you choose determines whether or not you need cell data connection. Currently standalone navigation software on WP7 is quite limited or non-existent. If that is important to you, you may want to look for other Smartphones like iPhone or Android phones. Your old WM software won't work in WP7.