I am wondering where all of the old Presidential limosines go once
they are taken out of commission (i.e. where is/are the car(s) that
Nixon/Carter/Regan/etc.) were escorted around in?  Is there a museum
somewhere that has them all?

Hi! Thanks for the question.

Yes there are three museums that house the limousines used by past
presidents. These are the Henry Ford Museum, the Ronald W. Reagan
Presidential Library and the Clinton Presidential Center.

 
"A display of presidential limousines spanning our nation's history
from the New Deal of Franklin D. Roosevelt to the assassination of
President Kennedy and the attempted shootings of Presidents Ford and
Reagan is one of the fascinating features of the automotive collection
at the Henry Ford Museum in Dearborn."

"The Henry Ford Museum has one of the truly great automotive
collections in the world and the presidential limousines, all
Lincolns, is an important component of it."

"Presidential limos a hit at Henry Ford Museum" By Richard A. Wright
http://info.detnews.com/joyrides/story/index.cfm?id=74


"It includes the last horse-drawn carriage used by presidents, ordered
put into service in 1902 by Theodore Roosevelt; the '39 Lincoln
"Sunshine Special" presidential limousine created for use by Franklin
D. Roosevelt and also used by Harry Truman, and the famed 1950 Lincoln
"Bubble-Top" presidential limousine associated most with President
Dwight D. Eisenhower, but actually built for use by Truman. It was
still in service into the John F. Kennedy administration."

"Ford museum preserves greatest moments in automotive history" By Richard A. Wright
http://info.detnews.com/joyrides/story/index.cfm?id=392


----------------
In 1983, 1993 to 2001, Cadillac began supplying the limousines.

"Cadillac supplied Presidential limousines in 1983, 1993 and 2001."

"Lincoln"
http://www.answers.com/topic/lincoln-automobile


Such limousines are now housed at the Ronald W. Reagan Presidential
Library & Museum and at the Clinton Presidential Center.

"Currently, the 1983 Cadillac Fleetwood limousine resides at the
Ronald W. Reagan Presidential Library & Museum in California while the
1993 Cadillac Fleetwood Brougham is located at the Clinton
Presidential Center in Little Rock, Arkansas."

"The predecessor to the current limousine was the 2001 Cadillac
DeVille limousine. The vehicle is still in service."

"George's new car - Cadillac ONE"
http://www.gizmag.com/go/3642/


Search terms used:
"Presidential limousines" "Henry Ford Museum"
Cadillac "presidential limousines" museum

I hope these links would be helpful to your research. Before rating
this answer, please ask for a clarification if you have a question or
if you would need further information.
                                                          
                              
Regards,                              
Easterangel-ga                              
Google Answers Researcher

Great answer and quick response!  Thanks so much.

Thanks for the kind words, the 5 stars and for the tip! :)

According to the Secret service, all presidential limos post the 1975
Continetal limo, housed at Henry Ford Museum, are blown up.  It is
hard to believe that the 1983 Reagan Cadillac Fleetwood limo met such
a bad end.  In keeping with the history and the man underwhich that
beautiful car served, it seems too far fetched to belive.  of them
all, I find it to be the most attractive with it's sleek lines and
high roof.  It was the most beefy looking of all the limos.  They say
that it is displayed at Reagan's library in California, but I think it
is probably a later prototype, much like the limo used in Clint
Eastwood's movie, In The Line Of Fire.esp. if what the secret Service
say is true about all limos are now destroyed, then Reagan's and
Clinton's limo are not genuine. Yatming has a 1/24 scale diecast
replica of Reagan's 1983 Cadillac limo, but it isn't an accurate
replica.  The roof is standard limo height instead of the tall roof
his limo had, and the wheels are too small.  The wheels need to be
larger and fill the wheel wells more.  The other replicas in the
presidential limosine line-up, esp. the Kennedy car, are very nice and
accurately portray the appearance of those historic cars, but the
Reagan Cadillac looks like they took too many short cuts and did not
actual research the vehicle in order to capture it's unique
appearance.  I am disappointed in it; hopefully, they will redesign it
in the future. Hope that helps you out.