Although apparently the author of the article isn't a big fan - kudos to President Bush for allowing local park managers to decide whether or not to allow mountain bikes within the park.  Note that this doesn't automatically grant Mountain Bike access throughout the park, but rather allows the local park manager who will be familiar with where such access is appropriate (away from endangered flora and fauna) to grant such access and generally manage that portion of the park.  In terms of comparison to something motorized, mountain bikes are significantly lower impact (many of the damages associated with bikers are found with hikers - the only one that would really be different imho is erosion impact)

http://nbcsports.msnbc.com/id/27183753/wid/18298287/?GT1=45002