You are most likely running pig_cassandra against the latest release of Pig, which has two jar files in it one with hadoop and one without hadoop. Your PIG_HOME is set to the root directory of your pig installation, which contains those two jar files. The existence of TWO jar files breaks the pig_cassandra shell script.
I've submitted a patch for this:
(Please vote to get it included)
Until that is committed, you can simply remove the jar file you don't want to use:
rm -fr $PIG_HOME/pig-0.9.1-withouthadoop.jar
That should fix you.