If we change Python 2.7.3 and 3.2.2 to force sys.platform to linux2 (instead of linux3) and use "linux" in Python 3.3, we will have 3 differents values of sys.platform if Python is built on Linux 3:

 - "linux3" on Python <= 2.7.2 or Python <= 3.2.1
 - "linux2" on 2.7.3 <= Python or 3.2.2 <= Python < 3.3
 - "linux" on Python >= 3.3

I don't see how it will help backward or forward compatibility... It's exactly as the current state (sys.platform == 'linux3' on all Python versions): applications have to use sys.platform.startswith() to work correctly on any Linux version.

Well, except maybe if you plan to write applications working only on Python >= 2.7.3? ... this version is not released yet.