Google have announced a brand new way of displaying PPC ad URLs from January 2011.
As a result of testing, Google identified that a standardised approach to displaying URL’s in PPC ads improved click through rates. During January, they will automatically switch all existing display URL’s to lower case.
e.g. If your display URL is is Subdomain.Example.com/Subdirectory, it will appear as subdomain.example.com/Subdirectory.