Seems like I've heard of something like this, but it's been too long to
remember much.  OTOH, you need to seriously consider what you're asking
for.  Depending on your location, your cellular service may be totally
dependent on just one tower.  Since no cellular provider can guarantee
service 24x7 at any given location, including yours, think about this
next time excrement occurs and dialing 911 becomes a possibility.

Here's a MERGE from Cidco http://www.cidcocom.com/merge/.  This one only
works for Motorola phones, there is the "Dock-n-Talk"
http://www.phonelabs.com/prd05.asp that works for many different phones.

We have the MERGE with a Moto v551 & T720.  Not connected to a landline (but
could be) and we have 4 extension phones (2 wireless).  Our 2 story home now
has phone access in all living areas, with no spotty connection.  Ours works
great!
-David

Try researching some of these links:
http://www.thehightechstore.com/cellinterface.html
http://www.cellantenna.com/Dockingstations/CellDock/celldock1000.htm
http://www.cashforcell.com/cellsocket.htm
http://www.phonelabs.com/
I haven't dealt with any of these retailers, but I have an older
CellSocket for my Nokia 6360 wired as line 2 in my home.

One more.  Uniden has added a bluetooth model to their line of multi-
handset models.  The basestation uses your cell phone via bluetooth.
See: http://www.uniden.com/product.cfm?product=ELBT595 
You can add up to 10 remote handsets.  I don't have that one but I do
have the standard 5.8 model with 5 handsets currently (2 Uniden and 3
Radio Shack branded) and really like them.  Especially the ability to
page all handsets for an intercom or transfer call--the old Casio system
I had you had to know where the person was to page them.

You might want to consider adding an external cellular antenna and maybe
even a signal booster to the cell phone.  That way you'd be insured that
the cell phone would get the best possible reception.

PhoneLabs Dock-N-Talk DNTA100 Universal Cellphone Docking Station
<http://www.anything4wireless.com/index.asp?PageAction=VIEWPROD&ProdID=31>

Cingualr USED to offer something called "Fast Forward" which was
basically a dock for your cell phone.  Then the phone was in the dock,
call would automatically forward to your landline phones.  It did not
incur airtime fees while your phone was in-dock, but did cost an extra
$2.99.  And of course, you still needed to have an active landline and
your land phones connected to it.

Not sure how popular it was.  Cingular may not even offer the service
anymore.