How much salt do potatoes absorb when being boiled?
From the linked question on pasta absorption, When cooking pasta in salted water how much of the salt is absorbed?, which basically says "the more salt you put in, the more will be absorbed".

However, one thing I've always thought to be true is that if you don't put enough in when cooking, you seem to have to add a whole lot more afterwards to lift the flavour to desired levels than you would have if you'd added it whilst cooking.
So the overall cooked vs table-added levels may in fact be lower if you add it early.

As it's cropped up in another answer - if you boil them with no salt whatsoever, the smell &amp; flavour are really quite different &amp; no amount of salt added after mashing can properly rescue them.
I can smell when someone's forgotten to salt boiling potatoes. My partner at one time wanted to believe it was because I can smell salt… erm, no. I don't think anyone can smell salt, but the potatoes smell completely different.

Late edit: The accepted answer on Ways to learn to season food correctly? would appear to agree with the statement that salting early is different to salting late - but I'd love to see some really solid evidence as to actually how that affects something as simple as boiled potatoes. All I have right now is 30 years of just knowing it makes a difference.