Some information for others is trying to write plugins for CKEditor 3.0.
I started by copying the source from the plugins / flash and now I got a button with the youtube logo .... this is an excerpt from plugins / youtube / plugin.js
editor.ui.addButton( 'YouTube', { label : editor.lang.common.youtube, command : 'youtube', icon: this.path + 'images/youtube.gif' });
You will also need to edit the language file ... for example. Lang / en.js
Add your plugin name to the "general" section (this displays as a tooltip when you hover over the button) and add an entire block for your plugin with all your lines, for example ...
// YouTube Dialog youtube : { properties : 'YouTube Properties', propertiesTab : 'Properties', title : 'YouTube Properties', chkPlay : 'Auto Play', chkLoop : 'Loop', chkMenu : 'Enable YouTube Menu', chkFull : 'Allow Fullscreen', scale : 'Scale', scaleAll : 'Show all', scaleNoBorder : 'No Border', scaleFit : 'Exact Fit', access : 'Script Access', accessAlways : 'Always', accessSameDomain : 'Same domain', accessNever : 'Never', align : 'Align', alignLeft : 'Left', alignAbsBottom: 'Abs Bottom', alignAbsMiddle: 'Abs Middle', alignBaseline : 'Baseline', alignBottom : 'Bottom', alignMiddle : 'Middle', alignRight : 'Right', alignTextTop : 'Text Top', alignTop : 'Top', quality : 'Quality', qualityBest : 'Best', qualityHigh : 'High', qualityAutoHigh : 'Auto High', qualityMedium : 'Medium', qualityAutoLow : 'Auto Low', qualityLow : 'Low', windowModeWindow : 'Window', windowModeOpaque : 'Opaque', windowModeTransparent : 'Transparent', windowMode : 'Window mode', flashvars : 'Variables for YouTube', bgcolor : 'Background color', width : 'Width', height : 'Height', hSpace : 'HSpace', vSpace : 'VSpace', validateSrc : 'URL must not be empty.', validateWidth : 'Width must be a number.', validateHeight : 'Height must be a number.', validateHSpace : 'HSpace must be a number.', validateVSpace : 'VSpace must be a number.' }