Found this -- from Wilton's Word and Phrase Origins http://www.wordorigins.org/thelist.htm (Book it, Maxo! )...this is the list page, not the home page.

"Break a Leg"

Superstition against wishing an actor Good Luck! has led to the adoption of this phrase
in its place. Popular etymology derives the phrase from the 1865 assassination of
Abraham Lincoln. John Wilkes Booth, the actor turned assassin, leapt to the stage of
Ford's Theater after the murder, breaking his leg in the process. The logical connection
with good luck is none too clear, but such is folklore.

There is no evidence, however, to suggest that this is the true derivation, and since the
earliest usage of the phrase dates to the 1920s, there is much to suggest that it is not. The
best that can be said is that the origin is unknown.

In the theatrical community it is commonly told that the phrase refers to bending the
knee, an old style of bowing. To break a leg was to bow a lot, meaning a successful
performance. But there is no evidence to support this explanation and the fact that it only
dates to the 1920s mitigates against it. Much more likely is that the phrase arose from a
simple superstition against wishing someone good luck.

Partridge's A Dictionary of Catch Phrases suggests that there may be a connection with
the German phrase Hals und Beinbruch, an invitation to break your neck and bones. The
German phrase is used by aviators and is equivalent to the English phrase Happy
Landings!. Both phrases arose about the same time, the early twentieth century, but the
connection between the German aviation community and American theater is unclear,
so they may be unrelated.

Partridge's A Dictionary of Slang and Unconventional English, published some eight
years before the other source, does not list the theatrical meaning. Instead, it lists an
obsolete meaning of "to give birth to a bastard child," from circa 1670.