Possible Duplicate:
Should you always use the accent in foreign words like “résumé”?
“Whereäs” as an alternative spelling of “whereas”
Is it spelt “naïve” or “naive”?
Merriam-Webster lists both spellings without any comment on validity / usage. The second variant seems to be the French original, and the other the "anglified" version.
Is there even a slight, maybe stylistic, difference?