One way of doing this would be with an HTML page that is the default browser homepage enforced by a GPO. Put IE in the startup folder and after login, the MOTD would be displayed.
Other way of doing it would be with a logon script, which displays a message box. It is easy and free, but perhaps not appealing enough.
The logon script could bet set in Default Domain Policy -> User Configuration -> Policies -> Windows Settings -> Scripts -> Logon, and should be similar to this (vbs):
<pre>WScript.Echo “This is the message of the day”