This is from my own experience today: I wanted to use my landline to call (sounds better; maybe psychological though) instead of talking to my n800, so on GC mobile Web site I changed my "mobile" number from Gizmo to landline. When I initiated the call from GC, only my landline ringed, not n800. So it did work, I think. After the call, I changed the mobile number to Gizmo again (this may not be necessary of course).
GC can route your calls to whatever # you specify for them. SIP phones, landlines, mobiles, Skype In. Whatever you want, as long as you add it to their list and choose it.
Grand Central Beta has been closed since around March of this year. Just keep checking the site. They should be going public soon, although won't be totally free as in Beta.
