According to a site I found on Maine Coon Polydactyls, President Theodore Roosevelt had a poly named Slippers, who was one of the first feline residents in the White House. Ernest Hemingway was given a polydactyl Maine Coon Cat when he lived in Key West, Florida by one of his drinking buddies, a sea captain named Stanley Dexter. There are many polys--descendants of Papa's--at his estate in Key West, which is open to the public.

Researchers taking censuses on polydactyl cats (now there's a job: "Hold on a minute, didn't we already count that black and white one over by the dumpster there?" "Jesus, Steve, hurry up and count the paws, he's scratching the hell out of me") found that there were greater populations in the Boston area than in New York City or Chicago. In Europe, polydactyl cats are rare because they were practically wiped out during medieval times due to superstitions about witchcraft (Kelly, Larson, 1993)

Polydactyly (Greek: poly=many, daktulos=fingers) is more common among New England cats, and some people say "mitten footed cats" originated there. Several sources I checked recounted the story you told, that ships' captains carried them onboard because they were considered lucky (and better mousers, one source said). An article from Cornell University's Cat Watch (1998) looked at studies done on polydactyl cats from the 1940s to the 1970s, and tentatively concluded that the trait probably initially occurred in cats who came over from England to the Boston area with the Puritans in the mid 1600s. There was also speculation in the article that the mutation might have developed in cats already in the Boston area, and descendants of those cats were carried aboard trading ships to the Halifax, Yarmouth MA, and Nova Scotia areas, http://www.straightdope.com/mailbag/m6toecat.html which now have sizable multi-toed cat populations.

Patient even towards children.

Ernest Hemingway
Between writing great literary works, like “The Sun Also Rises” and “For Whom the Bell Tolls,” 20th century American author Ernest Hemingway had time to keep several cats.  He was particularly found of polydactyls (cats with extra toes).  The Ernest Hemingway Home and Museum still houses over fifty descendants of his cats, with such lofty names as Ava Gardner, Emily Dickinson, and Pablo Picasso, as stipulated in his will.  As Hemingway said, “One cat just leads to another….”

http://www.boutiquekittens.com/Celebrity-Cats.263.0.html

In personality and care requirements, American Polydactyl Cats or Hemmingway cats as they are also known, are much like any other cat. Many owners who have a 'thumb cat' have made the observation, however, that their American Polydactyl cats seem to have a more relaxed and mellow personality than other cats they have owned. These cats are quite hardy and can survive even in snowy weather.  American Polydactyl Cats adapt very well to indoor or outdoor life, or a combination of the two.  They make outgoing pets, and are reported to be affectionate and patient even towards children.

In size, American Polydactyl Cats are medium to large with bodies strong and muscular in appearance. Thumb cathead is broad, with a modified stop, and medium in size. Mitten cat nose and muzzle is medium sized and the chin has a squared appearance. The wide-set ears have pointy tips. Occasionally, American Polydactyl Cats used for stud purposes have very well developed cheeks. The rounded eyes of American Polydactyls are set at an angle, and any color or color combination is acceptable. This is true also of coat color and fur length, so long as the hairs lie close to the body

http://www.prairielanecrescent.com/HereKittyKitty.html

What is Polydactyl? The trait for polydactyl comes from a dominant gene, and only a polydactyl cat can parent another polydactyl. Although the Maine Coon (whose ancestors are also thought to have immigrated to the U.S. aboard ships) is believed to have been the first breed to have this trait, polydactyl is considered a fault in that breed, and indeed in every other breed except the Pixie Bob, a relatively new breed.

So, rather than a breed, polydactyl is just a genetic trait, somewhat like the genetics for the tabby pattern. Instead of the normal 18 toes (five on each front foot and four on the rear) found on most cats, polydacts have six or more toes on the front feet, and sometimes an extra toe on the rear. (The cat depicted in the photo has seven toes in front,

"Mitten Cats.") However, unlike the popular tabby, whose pattern can be found in many breeds, breed registries frown upon the polydactyl. Why? According to Barbara French, although not inherently dangerous, there is a possibility that the extra toes could be malformed, either with two toes fusing, or with ingrown toenails.

Although not approved as "purebreds," their human companions adore playacts. Whether called "polydactyls," "Hemingway Cats," or "Mittens," these many-toed cats warm the hearts and hearths of those people fortunate to share a home with one

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http://cats.about.com/od/felinegenetics/a/polydactyl.htm