C++ and Java.  See CHANGES.txt for more details.

  bash-only features, and /bin/sh is not a symlink to bash on all systems.
* If an input file is a Windows absolute path (e.g. "C:\foo\bar.proto") and
  the import path only contains "." (or contains "." but does not contain
  the file), protoc incorrectly thought that the file was under ".", because
  it thought that the path was relative (since it didn't start with a slash).
  This has been fixed.

protoc
- New flags --encode and --decode can be used to convert between protobuf text
  format and binary format from the command-line.
- New flag --descriptor_set_out can be used to write FileDescriptorProtos for
  all parsed files directly into a single output file.  This is particularly
  useful if you wish to parse .proto files from programs written in languages
  other than C++: just run protoc as a background process and have it output
  a FileDescriptorList, then parse that natively.

C++
- Reflection objects are now per-class rather than per-instance.  To make this
  possible, the Reflection interface had to be changed such that all methods
  take the Message instance as a parameter.  This change improves performance
  significantly in memory-bandwidth-limited use cases, since it makes the
  message objects smaller.  Note that source-incompatible interface changes
  like this will not be made again after the library leaves beta.

Python
- MergeFrom(message) and CopyFrom(message) are now implemented.
- SerializeToString() raises an exception if the message is missing required
  fields.
- Code organization improvements.
- Fixed doc comments for RpcController and RpcChannel, which had somehow been
  swapped.