Exchange Server 2010 has a lot of new features and in this new version we can use HTML and AD attributes in a disclaimer Transport Rule.
In order to use an AD Attribute we can use %%attributeName%% and HTML code can be added as well.
We are going to talk about that here at MSExchange.org but you can have a sneak peak of this feature in this post: http://msmvps.com/blogs/andersonpatricio/archive/2009/05/01/personalizing-disclaimer-in-exchange-server-2010.aspx
About the Disclaimer Transport Rule the creation process is identical of Exchange Server 2007 and we covered this process in the following article http://www.msexchange.org/tutorials/Using-Transport-Rules-Creating-Disclaimers-Exchange-Server-2007.html
