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*Fun fact: J.P. Morgan and Picasso were suspected for the theft and Picasso was actually questioned then set free!
Mona Lisa (left) and its empty spot in the Louvre (right)https://encrypted-tbn0.google.com/images?q=tbn:ANd9GcTsK-x1YyzR9qQmazQuKGaotTsYcQly9_sceWOGweIvORxSsdtL8A
A New York Times headline from August 24, 1911, reported the investigation into the disappearance of the "Mona Lisa."
http://media.npr.org/assets/img/2011/07/28/article-.jpg?t=1312440019&s=2
http://media.npr.org/assets/img/2011/07/28/article-.jpg?t=1312440019&s=2 
The Washington Post published this picture as the Mona Lisa in 1911.
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At the time of its robbery, Olympia was estimated to be worth £2.75 million. I'm not sure if its market value has actually changed because of the theft, but it was covered in many newspapers and magazines, including the NY Times, the Guardian, and the Telegraph, so I would guess that its value would have gone up. The more exposure an artwork gets, the more popular and thus more expensive it is likely to become. Also, once stolen, a work of art is more desired than before.
René Magritte. Olympia. 1948.
https://encrypted-tbn3.google.com/images?q=tbn:ANd9GcRyxRFoF3NGagMA0dlQGjGEHbjjGctaK2DNFWr_mD2D8moC9D-gvQ 
René Magritte's Olympia in its original location
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Ellis, Richard. Chasing Hot Art. Inside the World of International Art Theft. CBC Radio, 13 Jan. 2012. Web. 14 Jan. 2012. <http://www.cbc.ca/day6/blog/2012/01/13/chasing-hot-art/>.
Lacayo, Richard. "Art's Great Whodunit: The Mona Lisa Theft of 1911." TIME.com. 27 Apr. 2009. Web. 14 Jan. 2012. <http://www.time.com/time/arts/article/0,8599,1894006,00.html>.
McNair, Carissa, and Charles Hill. "Art and Crime." The Art Business. By Iain Robertson and Derrick Chong. London: Routledge, 2008. 197-210. Print.
"The Theft That Made The 'Mona Lisa' A Masterpiece." NPR : National Public Radio. 30 July 2011. Web. 14 Jan. 2012. <http://www.npr.org/2011/07/30/138800110/the-theft-that-made-the-mona-lisa-a-masterpiece>.