I don't think there's a universally accepted answer to this question in math circles. It is now normal to consider equilateral triangles as isosceles (just as squares are considered parallelograms), but that wasn't always the case- two or three hundred years ago, an isosceles triangle was normally defined to have precisely two equal sides and angles - and I'd bet there are still people who go by that definition. This is not a distinction you will ever need to know about for the GMAT, but if you're curious, the Official Guide defines an isosceles triangle as one with 'at least two equal sides', so equilateral triangles are officially isosceles on the GMAT.
Nov 2011: After years of development, I am now making my advanced Quant books and high-level problem sets available for sale. Contact me at ianstewartgmat at gmail.com for details.
Private GMAT Tutor based in Toronto