When using the new layout controls for work items introduced with TFS 2010 the Process Editor will remove or change xml elements and attributes it does not like.
Editing a WIT xml-file with a xml editor adding a LabelControl or WebPageControl and importing it to TFS will work fine. When you open the file with Process Editor it will make "in memory changes" to the xml. There is nothing to notify the user that this happens. It appears as if everything works fine. If the file is closed without any changes by the user these "in memory changes" are not saved. If the user makes any changes the file is marked with a star in VS to notify that it has been changed. When the user saves his/her changes the "in memory changes" that Process Editor made are saved as well.
Basically this means that once you have done anything "advanced" with your WIT the process editor MUST NOT BE USED EVER AGAIN for risk of losing your changes!