For the action bars and the buttons associated with them, I'm not so sure. I'm thinking that at least one bar mod has bars that can be changed based on current actions or situations, but I'm not sure.
For the items, though, there is a mod called Autorack (
http://www.wowinterface.com/download...php?s=&id=6394 ) that you can set up to 8 slots from your bag to be shown based on combat status, but not on an individual basis; they all show or they're all gone if you're out of or in combat, respectively.
Sorry I'm not so much help about the bars, but maybe someone can read the way I phrased it and post some help.