Wikipedia's article on scimitars states the following, as of June 30, 2007:
Scimitars in history
In the form of the khopesh, the scimitar started playing a sometimes significant role in Middle Eastern warfare more than two millennia before the advent of Islam. Famed scholar and Egyptologist, Zahi Hawass asserts that the Egyptians of the 18th Dynasty (circa 1600 B.C.) used new weapons technologies borrowed from the Hyksos, including "the scimitar" as important tools in fostering Egypt's regional domination which characterized much of the New Kingdom period [Zahi Hawass, Tutankhamun And the Golden Age of the Pharoahs, Washington DC: National Geographic Society, 2005, p. 21-22]. Some might judge the Hawass' use of the term anachronistic but nonetheless this provides evidence for the use of something akin to the scimitar in well before the development of the Persian shamshir.