Animated films of "Old Possum's Book of Practical Cats" have been considered many times over the years, including adaptions of the musical. As yet, none have come to fruition.
Amblimation 1991 Edit
Under the direction of Steven Spielberg, who decided to set the show during the Blitz in London (1940-1941)
Luc Desmarchelier also blogs about the development
LA mid 1990s Edit
Hans Bacher mentions that a studio in LA was working on the Cats project, but was finally cancelled in 1997, and the Cats Movie as we know it went into production.