\catcode`\_=13
        {\catcode`*=8
         \gdef_{\ifmmode\let\next=*\else\let\next=\_\fi\next}
        }
%This makes _ into a normal underscore when used in normal text, but
%when you use it in mathmode it is still the underscore character. So
%you can write
%        some_identifier $= x_0$
%but not
%        $$some_identifier = x_0$$
%
%Which solution is most suitable depends on your application.
%
%                                        Marcel van der Goot
%                                        marcel@vlsi.cs.caltech.edu