This makes me very, very happy. I got Pig AvroStorage working on S3 from EMR.

grunt> REGISTER /me/pig/build/ivy/lib/Pig/avro-1.5.3.jar
grunt> REGISTER /me/pig/build/ivy/lib/Pig/json-simple-1.1.jar
grunt> REGISTER /me/pig/contrib/piggybank/java/piggybank.jar
grunt> REGISTER /me/pig/build/ivy/lib/Pig/jackson-core-asl-1.7.3.jar
grunt> REGISTER /me/pig/build/ivy/lib/Pig/jackson-mapper-asl-1.7.3.jar
grunt> 
grunt> DEFINE AvroStorage org.apache.pig.piggybank.storage.avro.AvroStorage();
grunt> a = LOAD 's3n://agile.data/again_inbox' USING AvroStorage();
2012-02-29 09:49:13,022 [main] INFO  org.apache.hadoop.fs.s3native.NativeS3FileSystem - Opening 's3n://agile.data/again_inbox/part-4-0.avro' for reading
grunt> describe a
2012-02-29 09:49:19,951 [main] INFO  org.apache.hadoop.fs.s3native.NativeS3FileSystem - Opening 's3n://agile.data/again_inbox/part-4-0.avro' for reading
a: {message_id: chararray,from: {PIG_WRAPPER: (ARRAY_ELEM: chararray)},to: {PIG_WRAPPER: (ARRAY_ELEM: chararray)},cc: {PIG_WRAPPER: (ARRAY_ELEM: chararray)},bcc: {PIG_WRAPPER: (ARRAY_ELEM: chararray)},reply_to: {PIG_WRAPPER: (ARRAY_ELEM: chararray)},in_reply_to: {PIG_WRAPPER: (ARRAY_ELEM: chararray)},subject: chararray,body: chararray,date: chararray}