Using this C code running as an exe on "2012-09-18-wheezy-raspbian.img":

	while (1)
	{
		bcm2835_gpio_write(RPI_GPIO_P1_13, 0);
		bcm2835_gpio_write(RPI_GPIO_P1_13, 1);
	}

the pin output was this:

See Also

http://codeandlife.com/2012/07/03/benchmarking-raspberry-pi-gpio-speed/