I came across an interesting article entitled Customizing Windows PowerShell. It's a little rambling, but it looks at some cool ways you can customise PowerShell's look and feel. It discusses customising the built-in display XML files to change the way PowerShell displays things. In order to make this work, the author looks at how you can re-sign these files.
As is pointed out by Lee Holmes in a comment, rather than edit (and re-sign) the default XML, create your own XML files, add your own formatting definitions, and then override the default ones. Lee points out a good resource: http://www.leeholmes.com/blog/DESCRIPTIONSupportInMonadPart3.aspx.