This is the inverse of AvroAsTextInputFormat (introduced in AVRO-808). AvroTextOutputFormat would be useful for streaming programs as it allows them to write an Avro Data File with a "bytes" schema.
Here's a patch with unit tests. Run the following before applying the patch.
svn mv lang/java/mapred/src/test/java/org/apache/avro/mapred/TestAvroAsTextInputFormat.java lang/java/mapred/src/test/java/org/apache/avro/mapred/TestAvroTextSort.java
This looks good and is a great addition. I've just committed it. Thanks, Tom!