Many fences in rural Arizona were put there to keep cattle in, not to 
keep people out.  Much of the cattle land in this state is range land 
that is essentially public lands, leased by ranchers from the 
government, for the purpose of  grazing cattle.  If  I'm on public lands 
and encounter a fence that isn't posted as Private Property or otherwise 
explicitly prohibits trespassing, I'll cross that fence in a heartbeat 
with no qualms about it.  I will, when possible, look for a gate or an 
area that can be easily crawled under or stepped over without putting 
stress on the fence.  On the other hand, if a fence is posted,  I  
assume that whoever posted the signs had a legal right to do so and I 
will not enter unless I have the proper permit to do so.  Many times the 
permit required is nothing more than a valid Arizona Hunting License, 
which I always have.   I guess my point is, if it's posted "Keep Out" 
then you should keep out.  If it ain't posted and you can reasonably 
assume it isn't private property, then it's fair game.  

-- Sprocket
 [100 Arizona caches in one month! (October, 2002)]