There's some discussion of this issue in
this thread on the Home Improvements forum.
Apparently GV can only port from a cell phone number, but some have done a two-stage port: first from land line to cell, then from cell to GV.
Edit: Oops, sorry, it looks like GV can port from a landline now. They didn't support that originally.
I use GV with the Obi100 and it's been great so far.