The Oil Drum: Australia/New Zealand

Although there have been suggestions to the contrary, the uranium whose fission provides the thermal in geothermal is not in the core but rather distributed in the crust.

Yes, geothermal is renewable (until the U and other unstable isotopes are gone), but the unfortunate timescale (see low heat flux stated above) means that what one is doing is actually mining stored heat. Yes, there is a lot of it (and in certain places it is rather close to the surface), but the very reason that the heat is stored at depth (low thermal conductivity of the rock) also means that harvesting the heat at an economical rate is a challenge. And if you solve that problem (perhaps using e.g. horizontal drilling), then you will nevertheless deplete the heat--you are limited by the thermal conductivity of the rock. Eventually, you will need to drill again elsewhere.