Just a quick experiment powering the Raspberry Pi using the 5VDC and GND pins via the 26P GPIO header. The Pi powered up with no problems including providing sufficient power for my direct attached Edimax EW-7811Un 150 Mbps Wireless 11b|g|n Nano Sized USB Adapter. I was able to remotely connect to the Pi using the WiFi connection and control the GPIO pins.
Please be aware that if you attempt this, you should use a regulated power supply. Don't connect a battery pack or unregulated power supply unless you are using a voltage regulation circuit. Un-filtered/unregulated power may cause damage to you Pi.
You can see the specific header pins used in the photo below:
- PIN 2 = +5VDC (VCC) (Red wire in photo)
- PIN 6 = GROUND (Green wire in photo)
* Raspberry Pi is a trademark of the Raspberry Pi foundation.