How do PCI Express Graphics Cards pull power from both the slot and the external 6-pin connector?
Most of this, as you've probably seen from the comments, isn't actually electronic trickery. They run multiple power districts across the board and drive them from the PCI-e slot and the cables separately. They're all running on a common ground from the same PSU, so it works quite nicely. Usually you'll see the VRAM and peripheral logic running from the PCI-e slot supply, and the GPU itself running from the PSU.

For boards like the GTX260 OC2, which are explicitly designed for overclocking, you'll see cases where two 75W 4-pin PSU connectors go into the same card. When more than one is used, they do use some form of current limiting (can't tell what on my card - it's under the housing) to "combine" the maximum current of the two inputs. This apparently results in a maximum consumption of 225W rather than 150W, which can be useful for people insane hardcore enough to cool their hardware with liquid nitrogen.

Rather surprisingly, a little research indicates that computer power supplies are designed in a way that allows you to directly wire two +12V rails together to get a boost in maximum current, without any damage to the supply. I'm skeptical, but it looks like some nutcases on various forums have done it, and one company even sells a special connector that combines them. If it's a legitimate thing that the ATX spec says they should allow, then I guess certain cards may just join the rails together in a similar way. I wouldn't recommend trying it yourself, though.