Yes, I agree Bob, that these "spoofs" are done every day in spam, even with this "lowercase the host" policy that most browsers seem to be using, in place.
It really has nothing to do with the DNS system, since the browser is never asking for a PTR RR. As I stated originally, it's for aesthetic reasons. It seems reasonable that the web browser should try to always preserve the casing received in the response Host header, e.g.
http://www.MyLongDomainName.com being more readable than
http://www.mylongdomainname.com.
-rory