I know most of it comes from pressure and tidal forces of the moon/sun, but I'm wondering if we know how fast this heat would generate? So if we somehow end up pulling greater than 10% of the heat from down there for several years will we reach a point of diminishing returns? and will there be any negative results from cooling that down?

I understand the center of the earth is huge, but if they're talking about pulling 2% of the heat out .... that seems like enough to do something....

The heat is created by the rotational difference and friction between the the planet's crust and its mantel. Unless Earth stops spinning, we will never run out of geothermal energy.

The numbers being bounced around here seem a little strange to me.

Geothermal heat is actually nuke; it comes from the decay of radioactive elements within the earth's crust. If you look at the earth's energy budget you will see that the total amount of energy on a yearly basis from geothermal is 23 terrawatts. This is over the entire earth's surface. The annual worldwide energy consumption 13 terrawatts, so if you captured a little more than 50% of all the geothermal you would have just enough to meet the world's energy demands. This is, of course, at 100% efficiency - hope all you want is heat: if you want electricity you'll get significantly less than this.

Consuming more than the steady state amount of heat evolved by the earth would be non-sustainable, but I'm not sure how much energy is stored in the earth - it might not matter that much.

Can someone point me to where they are getting these numbers? Where are the calculations and what are the assumptions?

Close, they're not pulling 2% of the heat out, the heat is getting wasted anyway, draining into the rest of the earth and cooling before it reaches the surface. We're simply tapping something being wasted anyway.

Andrew, keep in mind that the heat they're talking about accessing is only the heat close to the surface. In time, more will well up form below the crust to replace it. But yes, if we pull out heat faster than geology replenishes it, then the rock being tapped will get cooler, and we will not be able to extract as much energy. The total heat stored under our various nations is a battery that geology has charged, much like how fossil fuels are a battery the sun and dead plants have charged. I don't know the rate at which heat in the crust can be replenished. We can draw more than that, but if we do, then we have no right to call our geothermal energy renewable.

As for any more direct effects (I assume you are thinking of things like earthquakes and volcanoes) I have no idea.

I'm not quite following your comment, Cybercat. You seem to be saying that the amount of heat they are talking about would have been lost otherwise. But heat below the surface has only 2 options: it can either stay underground, or reach the surface (and eventually radiate into space). Your claim that the heat dissipates before reaching the surface doesn't make sense. The thermal energy would then still be in the crust, waiting to be released.

As for heat already reaching the surface- Yes, the farther down you go, the hotter it gets, and yes, some geothermal energy reaches the surface naturally, in the form of volcanoes, hot springs, and so on, but the quantities of energy the article is discussing are much, MUCH higher than the amounts that reach the surface in a year. They said 1% of Australia's geothermal potential would provided 26,000 years of power; that means 100% of it would provide 2.6 million years of power. If that amount of energy were reaching the surface naturally, Australia would shine as bright as the sun.

No, that figure for "potential" needs to be interpreted the same way we interpret figures like "250 years worth of coal reserves." The difference is that with geothermal, we start with a much larger reserve than we did with fossil fuels. And we can better quantify the rate at which the reserves replenish themselves.

So is the illustrated depth of about 4.5 km what you would need to do this anywhere, or does local geology weigh in?

Geothermal heat is caused predominately by gravity not radioactive decay, the difficulty comes with the investment of drilling so deep, most of Australia's "Hot Rocks" (those with enough heat to be geo-thermally viable), are rather deep.
Rather than any technical difficulty, Investment and political will are the problem. Generating such cheap energy in one phase, also generating this power from a large number of local plants as opposed to single large plants, cuts out all the fuel suppliers, transporters, large centralized electricity companies etc from the energy generation equation.
This is not economically attractive to governments and their wealthy supporters.
Looking at a country like Iceland, where the "Hot Rocks" are very close to the surface, they generate upwards of 90% of their electricity by geothermal, and are now investing to turn their whole economy over to hydrogen liquid fuels as well, because they have abundant cheap electricity with which to create the liquid fuel.
Geothermal is the number one option for carbon neutral electricity generation. When some one tells you geothermal is unviable because of the huge start up costs, compare the figures with the estimated costs of disposing NUCLEAR WASTE that has already been generated, not including the danger and future fuel use involved.