The tiny power brick that comes with the lapdock is a Motorola SPN5936A and it puts out 19 volts at 1.58 amps. If we just assume away a bunch of efficiency stuff, that's roughly 30 watts. The max power draw of the Pi is in the neighborhood of around 3.5 watts typical worst case (full load on CPU and GPU, but, not overclocked or overvoltaged). Subtract out the display, keyboard, trackpad, battery charger, and USB hub, and you're probably looking at somewhere around 10 watts left over, which would normally be reserved for USB devices plugged into the lapdock hub. At a generous 1 amp at 5 volts, two such devices would use up that remaining 10 watts. However, most devices draw 0.5 amp, or less, so, that leaves 5 watts for the Pi - plenty of headroom over the 3.5 watt peak load, which is about 12%.
Another way to look at it is that Joe_T has gotten 3 ~ 4.5 hours of use out of his Pi when it was plugged into the lapdock without its charger connected to the AC/mains. That included operating the lapdock display, keyboard, trackpad, and USB hub, so, since it has a 3,600 milliamp-hour battery, that suggests the Pi used somewhere around 700 ma / 3,600 ma-h, or about 20% of the power available from the battery per hour (I've simplified a few things to ignore internal battery resistance, various inefficiencies, etc.). So, 12 ~ 20% is well within the same ballpark.
When I've got cables splayed out while fitting everything into the under-dock case, I'll measure the actual drain out of the battery with everything connected, as well as the drain from the Pi under various compute loads, so I can subtract the pairs of values to determine what the load from the lapdock really is. 3 ~ 4.5 hours is more than I would have expected, given that the Atrix phones reportedly run for up to eight hours on the lapdock battery, and they have their own internal battery, too.
This really is too cool for school!
The best things in life aren't things ... but, a Pi comes pretty darned close!
"Education is not the filling of a pail, but the lighting of a fire." -- W.B. Yeats
In theory, theory & practice are the same - in practice, they aren't!!!