I've been doing it with Asterisk (specifically Trixbox) for quite a while.
I also buy a $30 sim card for where ever I am...then I call a local number (from
www.voxbone.com) and it rings me back. Then I have a dialtone just like I was sitting in my house in Virginia.
Its also nice for people who want to reach me....they can call my home and press "5", or what ever I setup, and then get transfered to my cell where ever I am in the world.