It is all possible on IIS. The question is what authentication
protocol and identity provider you want to use.

If you plan to use the ones built into Windows, then you will have to
use Domain\username or username@domain.com (assuming your UPN Domain
name = Domain name). That is the only secure approach.

Basic Authentication protocol is insecure and allows the Web Server to
prepend "Domain" when performing logons, thus allows you to provide
just username -- but only within one domain -- if you have >1 domain,
this approach won't work.

All other secured authentication protocols require username and
domain.

Custom Authentication Protocols in the form of 3rd party Addons can
allow just username, but you'll have to purchase/find them yourself.

My personal opinion is that users get used to username@domain because
that's how most popular and secure authentication protocols work. You
do a dis-service by trying to simply to a plain username.

//David
http://w3-4u.blogspot.com
http://blogs.msdn.com/David.Wang
//