I cannot reproduce it and I've never seen anything like it (Windows 7 64b and Vista 32b; Arduino 1.0).
Further testing appears to indicate that typing a line like this triggers it: #
Typing a # into an empty sketch causes triggers the symptom?
"string" is highlighted for you but is not for me. But that does not appear to be relevant; this is fine here...
#include <loop>
#include <setup>
void setup( void ) { }
void loop( void ) { }