Engine
Thanks for correcting me about Porsche engines. You are right. I double checked the parts catalogue last night and found that the manual and tiptronic engines share the same camshafts. So, yes, all P-engines do run in the same direction.
Transmission
I was confused by the following picture of the new 911 (rear-engined) PDK gearbox
because, as you can see, the teeth of the pinion in the pic is a RH. I now realise why- it's because the PDK is a 3-shaft gearbox. The input shaft turns in the same direction as the flywheel (anti clockwise by looking at the flywheel), the secondary shaft turns clockwise, then the pinion shaft, as it meshes with the secondary shaft, turns anti-clockwise hence the RH pinion.
In manual gearboxes, which is a 2 shaft design, the pinion shaft turns clockwise hence LH pinion.
The upcoming mid-engined PDK gearbox for the Boxster and Cayman will also have a RH pinion (I think!) but moved to the other side.
I don't know how a tiptronic works so I can't make comments about that.
Reverse engine rotation
The theory of the reversing the engine/transaxle (manual 2-shaft) rotation is still valid if one wishes to install an upright 915 or 930 in a mid-engined car if he doesn't wish to flip the standard ring gear to the other side.