LD,
As Revit MEP 2008 is not available on it's own, but only as part of the AutoCAD Revit MEP Suite 2008, there is no free trial for just Revit MEP 2008. You can, however, request a trial of the MEP Suite, which will include Revit MEP 2008.
http://usa.autodesk.com/adsk/servlet/oc/offer/form?siteID=123112&id=9224125
HTH,
Kyle B
Revit MEP Product Manager
Kyle Bernhardt
Director
Building Design Strategy
Autodesk, Inc.