Said nub sticks up at 30 degrees if it's a boy, and less than this if it's a girl.
So based on my best ultrasound picture, which is la:

...it's a girl. There's nothing sticking up!! Or maybe there is? The sticky outty bit is at the same angle as the spine. Which means girl. Argh. I wanna know!!
2 weeks time and I should know. Woot!