I believe the use of "~>" rather than "->" in Racket was set in #lang rackjure which adds in some of the Clojure syntax to Racket. Also amusing is their explanation:

> For example the threading macros are ~> and ~>> (using ~ instead of -) because Racket already uses -> for contracts. Plus as Danny Yoo pointed out to me, ~ is more "thready".

http://docs.racket-lang.org/rackjure/index.html#%28part._.Ba...

More thready indeed :-p