The data sheet for the Broadcomm 2835 chip used in the Rpi (including GPIO info) is hosted here on the Element 14 website in the "Technical Documents" section of the Raspberry Pi group. If you're reading this posting on a discussion page, look for "Home>RaspberryPi", click on the RaspberryPi link to get to the group home page, then look for Documents link, then the bottom posting of "Technical Documents for the Raspbery Pi". The GPIO info is around page 102-103 of the Broadcomm document. Note that the numbering scheme for the GPIO signals in the Broadcomm data sheet is the GPIO numbers relative to the CHIP. If you want to know where those CHIP numbers come out on the Raspberry Pi HEADER, you need to go to the Wiki that John mentions above.
However, note that several functions listed in the wiki diagrams with color shading (purple, yellow, blue) are in fact PARTICULAR "preferred" usages of SPECIFIC chip GPIO pins. The green "GPIO 0" pin of the Wiki layout is in fact the GPIO 17 on the actual chip, and (for example) the yellow "TX" header pin is in fact the chip GPIO 14 that has been programmed for the UART function. Any of the purple, yellow and blue "function" pins in the RPi header is in fact a GPIO pin all its own that can be programmed either for the function listed in the wiki (its "logical engineering use") or for some other function if you would rather use it that way. Thus, there are a total of 17 GPIO pins from the Broadcomm chip that are brought out to the header, and they are not all in a consecutive numerical order.
I am in the process of designing an expansion board and it has been somewhat confusing figuring out what the GPIO numbering situation is, because different people and sources have used different numbering systems, some of the early documentation and prototype boards were numbered and silkscreened wrong, and the Broadcomm peripherals datasheet that goes into the deepest level of detail on what the true situation is was only released a few weeks ago. Good luck figuring it our for yourself!
I believe we have been promised schematics at some point. They should be released soon now that RasPi is in production. With luck, the BCM2835 symbol on the schematics will clarify which pins are brought out to the header.
