Death Knight loots weapons as normal. Upon looting the weapon, the Death Knight has some ability to "DE" [or disenchant] the weapon, granting them an essence of sorts that they then apply to their runeblade. Runes should probably still only work on the runeblade itself, preventing the DK from using the other weapons without first DEing and applying them to his own Runeblade. Essentially, the loot game and itemization is still in place, but the Death Knight is able to maintain a consistent runeblade. They simply absorb the powers of other weapons into their own blade. Each time the absorbing happens, the runeblade is 'wiped clean', just like gems... the new stats would replace the old stats, they don't add to them.
IE, you loot Lionheart Blade, DE Lionheart Blade, apply Lionheart Blade Essence to your Runeblade, your Runeblade maintains the looks you chose at the beginning of the game, but now has the stats of Lionheart Blade.
Unfortunately, though, it looks like Blizzard doesn't have any plans to let death knights form such a relationship with one blade over their entire career (though perhaps they only read the post before that favorite suggestion was edited in). As Salthem
We haven't fully decided on this at this time, we will try things out in the beta, but at the moment we are thinking that death knights can probably inscribe any weapon with runes.
Personally, I like this idea too, but taking weapon drops out of the equation for death knights removes a big chunk of the itemisation side of the game from them - it would have to be something pretty special to replace the feeling you get with your first epic weapon.
Not to mention the awkwardness this could cause in balancing loot tables as Ilsevele pointed out.
Do you think death knights should be able to have one blade permanently bonded to them? How do you think a sensible character progression could work out if that were the case? Or, on the other hand, do you think it would get boring just using the same weapon graphic for ever without being able to mix it up with newer, bigger, better weapons over time?