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I'm currently cycling a 29g which contains a piece of Malaysian driftwood. Normally, my pH out of the tap gasses out to 8.0, which is what my other tank has been running at since the start (no driftwood). After a week of cycling, I checked the pH on the 29g and it's at 7.0. I'm also using a 50/50 flourite/gravel mix for substrate.

I know that the tannins in the driftwood are probably the reason for the different pH, but the thing I don't understand is that my KH is still at 5.0 and my GH is at 9.0. I thought that the KH buffer needed to be used up before a pH change could take place.

You can change the pH regardless of what your hardness is, it just will take more of a kick to change the pH if the water is hard.

Your driftwood, the clay in the flourite, and the decaying organic matter from cycling most likely made the pH drop to what it is now, but it will be harder to change it from 7.0 now.

I'm just a little confused by all the threads I've read on this site about using up the KH buffer in order to make pH changes. From my experience that doesn't mean lowering the KH to 0 because my tank with a pH of 7.0 has a KH of 5. This is a couple of degrees lower than my other tank running a pH of 8.0, but not zero.

Should I expect my pH to rise again after I start doing water changes and the tannins are all leached out?

Yes, in theory anyway. It really depends on what you have going on in your tank. Without knowing if you are running plants, co2, what rate of decayed matter is in your tank and so on it's pretty hard to speculate. In a perfect pristine tank water changes will swing the PH back up towards a middle ground, however, few tanks are perfect or pristine. As for buffering, buffering is simply a way of stating how much resistance your water puts up to being shoved around. You try to push the ph one way, the water pushes back. The lower the kh and dh the less resistance the water puts up to the changes. This is of course way over simplified but it gives a simple visual. It's not always true that a low kh or dh will result in a low ph, you can have a low kh and or dh and still have a high ph and vice versa. The kh and dh (over simplifying here) relate to how stable the water will be when it does reach the ph level it will rest at.
I hope this helps, I know it's incomplete and all but it's a bit difficult to explain how buffering works without getting technical.

Thanks for the info. I guess I'll just accept what I have as far as the pH level and see how things develop as the tank cycles and I start water changes. Guess I was just shocked at how much lower this tank's pH is and the dramatic effect the driftwood and flourite had.