[Makubesu]

Now I know that buffers resist change in pH, and I understand how you make one.  My question is, though, when it resists the change does the pH not change at all until the buffer capacity is reached?  Or does it change very slightly until capacity is reached?  

[AWK]

Your last thought is (almost) correct

[Donaldson Tan]

changes in pH is negligble as long the buffering capacity isn't exceeded.

since the changes are negligible, the pH is often approximated as constant.

you can always do the math to justify this. the only one thing that truly holds constant is Ka or Kb (depending on the type of buffer) at constant temperature.

[Makubesu]

I have a question about that than.  Lets say I put in an insoluable salt, like magnesium hydroxide in a buffered solution with a pH of 7.  To figure out the concentrations and everything you'd use the equation:
KSP = [Mg+][OH-]^2
which in turn leads to:
8.9 x 10^-12 = (X)(2x + 1.0 x 10^-7)^2

Now because it's a buffered solution, that should mean that the concentration of OH- should remain about constant.  That would mean that you could right away ignore the 2x because it shouldn't be able to do a significant change right?  But what if the value of the KSP was such that x would be a number that would greatly change the 1.0x10^-7?  Do you still ignore the 2x?

[Borek]

Precisely speaking any addition of acid or base changes pH of the buffer solution. Thus pH is not constant - all you can say is that you are on the flat part of the titration curve before endpoint, so the changes are very small. Whether they are negligibile depends on how do you define 'negligibile' - 0.001? 0.1?

The correct answer is that pH changes very slightly near the pKa, and it changes the faster the farther you are from pKa.

Check out this buffer capacity lecture for further details.