cdata, javascript and xhtml1.1

Could someone expand on why CDATA is essential?

our page* validates fine without it, yet when I include it the page no 
longer appears to load :-( ie5.2 mac mozilla20021115 mac ie6 pc

the 1 line short script hides images whilst they load, perhaps someone 
knows a better method?

thanks

Jonathan Chetwynd

* http://www.peepo.com/xhtml