OWA 2007 provides the option to pre-configure the Default Domain in the Exchange Management Console (EMC) so users do not need to enter it.  With this configuration, users only need to enter the logon name and password to authenticate via the Exchange Forms Based Authentication form.  To predefine the default domain,

1. Open the EMC and navigate to Server Configuration\Client Access and click on the Client Access server.
2. Right-click on the OWA folder and select Properties.
3. Move to the Authentication tab
4. Under the Use forms-based authentication option, select User name only.
5. Enter the domain name in the logon domain field.
6. Press Apply and OK to save the changes.
7. Reset IIS by running iisreset /noforce from the run command prompt.