At risk of becoming the most hated man in all Haskelldom, I'd like to suggest that the Haskell logo not use lambda symbols. Or at least not as the central element. Sorry, I know I'm late to the party, but the thing is there is nothing distinctive about lambda; it's common to all FPLs. Besides, Lisp/Scheme already have that franchise.
What is distinctive about Haskell it's use of the monad. The Pythagorean monad symbol is wonderfully simple:
http://en.wikipedia.org/wiki/Monad_(Greek_philosophy). Something might also be done with the triad to reflect the fact that the monad in cat theory is actually a triple (http://en.wikipedia.org/wiki/Triad_(Greek_philosophy)).
The "Cup or Monad" (http://www.sacred-texts.com/chr/herm/hermes4.htm), with a little bit of work, could be turned into an amusing Haskell manifesto. Lots of interesting imagery from the gnostic tradition (http://www.sacred-texts.com/gno/th2/index.htm), although SICP seems already to have used something similar (http://mitpress.mit.edu/images/products/books/0262011530-f30.jpg). Hermes Trismegistus = Thrice-great Hermes -> Haskell Trismegistus, Thrice-Glorious Haskell, etc. Might be too cute.
There's also Leibniz' monadology - I can't think of any visual imagery to go with it, but he did end up with the "best of all possible worlds" hypothesis, which gives us a slogan sure to irritate: "Haskell - the best of all possible languages". Not to mention "Haskell Monadology" as a name for the official Haskell definition, etc.
Hey, at least it isn't cute.
-gregg