In reply to Removed User: It would depend mostly on the friction between the back wheel and the road. I find the limiting factor on climbs like Hardknot pass is either keeping the front wheel on the ground when I sit down, and stopping the back wheel spinning if I stand up.
Both these things tend to start happening at around 25-30%. If there was more friction from a thicker back tyre or a really grippy road you could stand up, lean forward and then the limiting factor would be your gears and leg power.
I would imagine my limits are around 25% on a fairly slick wet road, with normal winter tyres on a road bike. 30-35% sustained in good conditions with normal tyres. I don't have a clue about if I was to change the gearing and stick on a mountain bike tyre on the back - short sections of 50% perhaps?
It would be pretty tough to find out though because you would have to first build a new road or ramp to test it. Steepest at the moment is thought to be Baldwin Street at 35%.
http://en.wikipedia.org/wiki/Baldwin_Street