You can edit the front page to automatically populate the domain/ part for the username....
Well i did at any rate 
Here's how:
Take a look at the file logon.asp in notepad [or in a program that won't save formatting hehe] - the file is located in a default install at:
C:\Program Files\Exchsrvr\exchweb\bin\auth\usa
search for the following:
Code:
<% If g_fIsMSIE5Rich Then %>
((( Before you change anything, backup any files you change so you can restore them should things not work out for you )))
and change this:
Code:
<% If g_fIsMSIE5Rich Then %>
<BODY scroll="AUTO" bgColor="#3D5FA3" text="#000000" leftMargin=0 topMargin=0>
<FORM action="/exchweb/bin/auth/owaauth.dll" method="POST" name="logonForm" autocomplete="off">
<% Else %>
<BODY scroll="AUTO" bgColor="#FFFFFF" text="#000000" onload="setFocus()">
<FORM action="/exchweb/bin/auth/owaauth.dll" method="POST" name="logonForm">
<% End If %>
to:
Code:
<% If g_fIsMSIE5Rich Then %>
<BODY scroll="AUTO" bgColor="#3D5FA3" text="#000000" leftMargin=0 topMargin=0>
<script Language=javascript>
<!--
function logonForm_onsubmit()
{
if (logonForm.username.value.indexOf("@") !=-1)
{
return true;
}
logonForm.username.value = "DOMAINNAMEHERE\\" + logonForm.username.value;
return false;
}
//-->
</script>
<FORM action="/exchweb/bin/auth/owaauth.dll" method="POST" name="logonForm" autocomplete="off" onsubmit="logonForm_onsubmit()">
<% Else %>
<BODY scroll="AUTO" bgColor="#FFFFFF" text="#000000" onload="setFocus()">
<script Language=javascript>
<!--
function logonForm_onsubmit()
{
if (logonForm.username.value.indexOf("@") !=-1)
{
return true;
}
logonForm.username.value = "DOMAINNAMEHERE\\" + logonForm.username.value;
return false;
}
//-->
</script>
<FORM action="/exchweb/bin/auth/owaauth.dll" method="POST" name="logonForm" autocomplete="off" onsubmit="logonForm_onsubmit()">
<% End If %> and change the DOMAINNAMEHERE to whatever your domain is that the users reside on.
Now, open up the IIS Manager under Administrative Tools and right click on the Exchange website found within the Default Web Site tree....
You need to go to the properties of it and replicate what I have in the attached screenshot...
i.e. [in case the picture doesnt appear] on the Exchange properties window, go to the Directory Security tab and click on Edit... under the section Authentication and Access Control.
On the window that appears, you should change the default domain and realm to a single backslash.
Job done 
Cheers
Nath.