In a pressurised aircraft you can have a feed into the tanks without a pump!
Jason has it right - Turtle Pack. These are an excellent product, and tick all the boxes for safety (for as safe as one can be carrying fuel in the cabin!). I have tested and approved them for several ferry installations. That said, if you are thinking to pour the contents into the wing, rather than pumping into the system, the Turtle Pack could be a bit awkward.
Presumably, one would not use a pitot tube with a "bladder" type of ferry tank, because it would just keep it fully inflated the whole time?
Only if the dynamic (pitot minus static) pressure would be more than the weight of the top of the bladder itself. I have no idea how to calculate that. But I agree - why would you?
In fact, it's probably marginally safer not to vent a bladder type container to the atmosphere. A container filled with 100% fuel cannot ignite (due to a spark from static electricity) as there is no oxygen available.
And looking through the Turtle Pac website, I don't see any pitot or static connections anyway.
I guess this means that one should only carry 100% filled jerry cans as liquids are not compressible and thereby don't expand when ambient pressure decreases.
True, but on the other hand if the temperature rises, liquids will expand and if your container is filled to 100% with a liquid, there is no space to expand to. So the container will burst or at least start to leak through the seals.
So I'm not sure whether filling to 100% is a good idea.
My gut feeling tells me that you should have your containers filled to about 90% (so there is room for thermal expansion of the fuel, but there is not a lot of air to expand in case of pressure changes due to altitude) or 0% (means completely empty, and then you can just leave the caps off). Worst case scenario would be a container filled to just 50%. But that's gut feeling only, not backed up by real life experience or calculations.
And if I may make a stab at the reasoning why 90% is better than anything lower: If your container is filled with 90% liquid, there's only 10% gas that can potentially expand. Suppose that the ambient air pressure drops by 10%. To fully equalize this, the container only needs to "bulge out" by 1%. (10% x 10% = 1%). Whereas if your container is 100% gas, it needs to bulge out by the full 10% to fully equalize. 1% bulging out is something that I think most containers can handle, looking at the structure and stuff like ridges and small bulges in the side walls, but 10% not. (And I agree that full equalization is not needed, as the jerrycan is supposed to withstand some pressure differential. But the principle still holds.)
Anyone with more practical experience and/or theoretical calculations?
As I always fly with pee bottles 
I am in agreement re keeping a little bit of air on there.
Avgas expands at 0.1% per degC and one could easily see a 20C change i.e a 2% volume expansion. With negligible compressibility, the force available to burst a solid container (with no gas buffer) will be massive.
As regards how much gas space should be left in, consider the worst case temperature change. Say the post-fill volume increase of the liquid is 2%, and if there is 4% gas in there (i.e. you would see a rise in the liquid volume from 96% to 98%) the pressure in the container will double (PV gas law). Doubling the sea level pressure amounts to 15psi net pressure inside the container, which is quite a lot. My guess is that plastic cans expand a lot anyway so are OK even if 100% filled, and anyway any rectangular container obviously has a lot of volume elasticity.
As I always fly with pee bottles I am in agreement re keeping a little bit of air on there.
The relative advantage that pee bottles have, of course, is that they are typically filled when you're flying at altitude. So they will need to deal with an increase in ambient air pressure eventually, not a decrease. Which is a lot easier.
On the other hand filling a pee bottle to 100% capacity may be a messy affair and for that reason alone I'd stop at about 75% (if able). ;-)
This post is a response to both this thread, and the other thread about 91UL...
Yes, I have practical experience carrying fuel in jerry cans. I avoid if at all possible. Only the very best quality cans don't leak a little, and the pressure changes just aggravate this. I have ferried with jerry cans, where there was no alternative - long legs in an MD500 helicopter and a few floatplanes, where it was necessary to land mid leg and refill.
The airfield where I used to keep the 150, had no fuel, so jerry canning was the only way to get fuel into the plane, unless I flew it away for Avgas. I got used to hefting and pouring 25l jerry cans into a high wing aircraft, but it was never a good idea - I spilled a lot over the years. Then, and still now, it is rare to find an airfield which sells Mogas around here. I don't know what others do, but I have a proper 1300l Mogas tank here. It is diked, grounded, has a pump and filter, and remote emergency shut off.
It is wise of regulators to limit the amount of fuel being transported in jerry cans. In a collision they are a hazard in many ways. I see people filling them in the back of their pickup trucks at the filling station - against the law, and a real hazard, as they are not grounded.
There was talk of ethanol, I'll reply that on the 91UL thread....
Suggest you also check (if only for your own info ;-) ) the legalities. I only ever once wanted to do this and that was in Nambia where it is a BIG no-no. There's even a section in the local air law dealing with this. Guess too many people are tempted / doing it anyway there, as Avgas is an issue, to put it mildly.
