This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
I'd say the biggest pre-requisites are time to play around and a willingness to learn. You don't have to know Java to use the basics of Pig. Some more advanced features (like writing User Defined Functions) require Java or Python. Even when Java is required it does not require knowledge of the full J2EE APIs. Having experience with some data processing will be helpful (e.g. SQL or use of ETL tools), as will experience with Hadoop. But neither are necessary.