I know that Roosevelt's condition was hid from the American public. But how well kept of secret was it? Did most politicians know that FDR couldn't walk? I imagine that at conferences with world leaders it would be obvious to foreign heads of state. Thinking about it, I feel most photos of FDR, Churchill, and Stalin are shown with all three sitting down.
I can't answer for Stalin and Churchill. But when FDR met with King Ibn Saud of Saudi Arabia, he was in his wheelchair and this exchange occurred:
King Abdul Aziz started kidding Roosevelt about, "You're lucky you're in that wheelchair. You can wheel yourself anywhere." And Roosevelt said to him, "If you like this wheelchair so good, I have an extra one. Here, I'll give it to you."
Source Frontline- http://www.pbs.org/wgbh/pages/frontline/shows/saud/etc/script.html
Churchill and Stalin definitely knew. Churchill stayed with the Roosevelts in the White House on a trip to the U.S. and Roosevelt stayed at the Russian compound during their meeting at Tehran. Most members of congress also knew of Roosevelt's disability although FDR had many ways to minimize people seeing him being "handled like a sack of grain" by Secret Service agents who helped him out of his car, across barriers, etc.
EDIT: Just had a chance to check my bookshelf. Hugh Gregory Gallagher has a whole book, "FDR's Splendid Deception" (Vandamere Press, 1999), which goes into great detail of FDR's efforts to conceal his disability from the public. It's an interesting read that gives some insight into the way FDR looked at his own inability to do certain things.
Antony Beevor (I think) writes about Churchill and FDR going for an evening walk during the Yalta conference. Churchill pushes FDR along a rocky track and they come to a low flat roofed storage shed. They decide to climb it to get a better view. It takes 20 minutes as Churchill pushes, hauls and drags FDR onto the roof where they sit and watch the sunset.