I am using ubuntu 12.02 32bit and have installed hadoop2.2.0 and pig 0.12 successfully. Hadoop runs properly on my system.
However,
I've resolved it in another way.
I got the same problem on CDH4.4 & Pig 0.11.0 when my Pig script was invoking a UDF from my java project which was compiled using Maven.
I've visited /usr/lib/pig/conf/build.properties file. Verified the versions mentioned against hadoop-core, hadoop-common, hadoop-mapreduce properties. Ensured that all these artifacts with same versions are included as dependencies in my java project's POM.xml file.
( Infact hadoop-mapreduce has 6 artifact Ids as per http://www.cloudera.com/content/cloudera-content/cloudera-docs/CDH4/4.2.0/CDH4-Installation-Guide/cdh4ig_topic_31.html. I've included all of them in the dependencies list).
After building my project's jar file with these POM settings, Pig script was able to invoke UDF without any problem.