Depends on if you are going on state or federal land. Firearms are prohibited in National Parks, such as the Boise National Forest, which is managed by the US Forest Service, part of the USDA. This was originally intended to protect wildlife from poachers and unlawful hunting. After some searching I was able to find the actual law restricting firearms, found at this link
http://edocket.access.gpo.gov/cfr_2002/julqtr/pdf/36cfr261.58.pdf
§ 261.57 National Forest wilderness.
When provided by an order,
the following
are prohibited:
(a) Entering or being in the area.
(b) Possessing camping or pack-outfitting
equipment, as specified in the
order.
(c) Possessing a firearm or firework.
(d) Possessing any non-burnable food
or beverage containers, including deposit
bottles, except for non-burnable
containers designed and intended for
repeated use.
(e) Grazing.
(f) Storing equipment, personal property
or supplies.
(g) Disposing of debris, garbage, or
other waste.
(h) Possessing or using a wagon, cart
or other vehicle.
[42 FR 2957, Jan. 14, 1977, as amended at 49
FR 25450, June 21, 1984]
The Department of the Interior is trying to change this however by writing a new bill to make it legal to carry a firearm in a National Park as long as it is legal for you to do so in the state where the park is located.
http://www.regulations.gov%2Ffdmspublic%2FContentViewer%3FobjectId%3D090000648053d422%26disposition%3Dattachment%26contentType%3Dhtml&ei=e0imSLP8BoLwsAOXkc3JDg&usg=AFQjCNGinEJAowSFmqtYf4yYhgxcckEntg&sig2=J488wCMJt7N15WggbewFFw