That's a difficult question to answer, but 99% of all frames are manufactured in China, Taiwan, and a few in Vietnam. Many companies assemble frames or components in their USA facilities, but they do not actually manufacture them here. This is true for aluminum bikes....carbon fiber, titanium, and steel frames have more in-country fabricators.
To my knowledge, Intense Cycles is the only 100% USA-made frame....they use domestic aluminum tubing from Easton and everything is made in Temecula, CA.
In the specialty road/mtb category, there are a lot of boutique frame makers in the USA, as well as some high-end bmx'ers. Nearly all of their steel tubing comes from overseas, however.