See http://universaleditbutton.org
Looks like a neat idea... until Atlassian implements it, here's a user macro...
I was inspired, so it just took like 30 minutes to figure it out 
Enabling the Button
- Create a User Macro (Administration -> User Macros) and call it universaleditbutton
- uncheck Macro has body
- select Macro generates HTML markup
- Here's the body of the macro:
#if ($permissionHelper.canEdit($action.RemoteUser, $content)) <link rel="alternate" type="application/x-wiki" title="Edit" href="$req.contextPath/pages/editpage.action?pageId=$content.id"/> #end
It should look like this:
Now you have to edit the page.vmd and blogpost.vmd decorator to include anywhere after "#set ($helper = $params.get("helper"))":
$helper.renderConfluenceMacro("{universaleditbutton}")