Er, no. Weissbier wasn't necessarily brewed from wheat, though many Weissbiers did contain some. The name derives from pre-industrial malting techniques. There were two methods of preparing malt:
- air-dried "Luft-Malz" which was pale in colour
- kiln-dried "Darr-Malz", which was dark in colour
Beer brewed from Luft-Malz malt was called Weissbier("White Bier") because of its pale colour. That brewed from Darr-Malz was called Braunbier ("Brown Bier") for pretty obvious reasons. Lichtenhainer is a good example of a Weissbier that didn't necessarily contain any wheat.
You may have heard another explanation: that the "Weiss" in "Weissbier" somehow comes from "Weizen", the German word for wheat. I don't believe it for a minute.