hi I wrote an artikel in myblog, its about a simple plugin for gimp to export layers as plists,
it is handy if you want to generate plists in windows or linux
hope it is usefull
http://www.mets-blog.com/cocos2d-x-gimp-plist-plugin/
download link: http://sourceforge.net/projects/gimpplistplugin/files/gimpplist.zip/download
UPDATE:
Heres one more small plugin that will pack your layers, its pretty straight forward can use some improvements
download link:
https://bitbucket.org/metinlapo/metsblog/downloads/cocos2dSpritePack.zip