no need for underscore.sty -- latex.ltx provides \textunderscore and \_ already;

--- a/lib/texinputs/isabelle.sty	Fri May 06 17:52:08 2011 +0200
+++ b/lib/texinputs/isabelle.sty	Fri May 06 20:25:41 2011 +0200
@@ -187,7 +187,6 @@
 }
 
 \newcommand{\isabellestyleitunderscore}{%
-%requires underscore.sty
 \isabellestyleit%
 \def\isacharunderscore{\_}%
 \def\isacharunderscorekeyword{\_}%