http://lists.blender.org/pipermail/bf-committers/2010-August/028306.html
Image:
http://www.pasteall.org/pic/show.php?id=4858
2.5 plugin capability takes another big leap with the ability for plugins to define their own contexts.
This should be useful for just about anything that would otherwise add too many buttons to the tool-shelf or an existing context.
Campbell has said before he wants to see Python defines spaces in the near future, if that gets added as well then we might see significant plugin capabilities in the months leading to 2.6.