Jon, the minimum version of calibre for a plugin to work is already supported. The discussion that BetterRed linked to was a suggestion that a maximum version of calibre supported by a plugin be added similar to the already supported minimum version found in the __init__.py file in the plugin .zip container.
Code:
name = 'Baen'
description = 'Downloads metadata and covers from Baen'
author = 'Grant Drake'
version = (1, 1, 0)
minimum_calibre_version = (2, 0, 0)