The design on the Shimano hubs is very simple: seal rings keep (most of) the dirt and water out.
You can check out some exploded views of their hubs at http://techdocs.shimano.com . I'm surprised to see the XTR FH-M970 only has one seal ring (non drive side), while the LX FH-M580 has three seal rings (left & right hand seal rings and a 'sub unit rubber seal'). Does the XTR hub compensate this somehow or does it need more maintenance?