This is a question I've pondered myself...
Some people claim that, "I was told/saw/explained by an ADCOM that GPAs are numerically adjusted x1.1/+.2 for more difficult schools". While GPAs from different institutions may be evaluated somewhat relatively, there is no way that a numerical adjustment is made.
Imagine that adjustments were made. What schools get additional points? Top 50? Then what about the drop from school 50-->51? Maybe a sliding point scale makes more sense, #1 Undergrad +.3, #2 Undergrad +.295, etc. How is this order established? By US News lists? I just can't imagine any system that would accurately serve the purpose of calibrating schools.
I've taken courses through various universities, and IMO, there is a wide variance in the difficultly of achieving top scores. However, I haven't felt any correlation between US News rankings and difficulty.
What I think happens: If you get a 3.5 from a top school, you are worse off than someone with a 3.6 from a lesser esteemed school. 3.5<3.6. However, for boarderline (GPA-wise) applicants, I would guess that consideration will be given to those from difficult schools. Note: This is my own personal speculation.
Edit: To the dude above attempting to distinguish between UCs and Cal States: It doesn't matter. UCB, UCLA, etc. are still state schools. They are funded by the state, are required to take x% in-state, as well as any state students graduating in the top 9% or something of their high school class. Yeah, you might not want to brag about going to Cal State Fullerton, but no future employer will go nuts when you tell him/her that you graduated from UC Santa Cruz.