I already keep a tablet beside the home phone, for using with DialCentral. Dial the (long distance within N.America) number on the tablet screen, and the phone rings. Talk for as long as you want, for free.

Also, I have two N800s, and it is fairly straightforward to use Internet Call to have a rudimentary video "baby monitor"... I would love to see this developed further, so that you have a full video security cam / monitor / intercom system...

EDIT:

All the 3G devices should "call home" every so often with their GPS locations, so that anyone in the household can see where all the devices are at any given moment. Would also make tracking device thieves (and errant teenagers) easier...

EDIT2:

"Family Home Page" has (1) current locations of all mobile devices (2) current calendars of all family members synched from various locations.

The Home Page is on permanent display in the kitchen in the home, but can also be accessed by any mobile device (or other screen in the house).

The home has an Asterisk PBX, and all mobiles are on this PBX. All calls to family members are to a single phone number, with a one or two digit extension number. When the devices are on charger or the person is in their room, all calls are routed to the person's home extension. When that person is out of the home, calls are routed to mobiles. Voice / video messages are stored on the central server and can be accessed anywhere.