His original name was actually Guillaume de l'Hospital; French spelling reforms later did away with a number of cases of silent 's' (which had been silent for a long time already), replacing it with a circumflex over the preceding vowel.

See: http://en.wikipedia.org/wiki/Use_of_the_circumflex_in_French...

and: http://en.wikipedia.org/wiki/Guillaume_de_l%27H%C3%B4pital#c...