All moves when used and hit a person for damage will get weaker each time in both damage done and knockback. At first it reduces quite quickly, then it only reduces a little bit each time, up to the point where I think it reaches a minimum after 8 times you hit with it. Something similar existed in melee, but in melee only B moves had reduction in knockback, A moves stayed the same knockback whenever. It also seems to happen at a much steeper rate at first. After 10 hits of anything else, the knockback and damage of a move is reset. The same thing happens after you die.
As for Samus her charge B is really in no danger because the actual need to use a fully charged shot for knockback twice in a row with less than ten hits that connect in between seems unlikely. A lot of people's KO problems right now are actually the fact that they don't realize that this dimishing effect exists on knockback and they abuse a good knock back/KO move before they need it. I think Samus has an advantage here, because of her projectile game/Zair and the like, she can use those kinds of moves as a battery and quickly rack up 10 hits from those and be good to go elsewhere. The same should go for characters who can hit with a lot of neutral A jabs.
This stuff is easily one of the biggest new additions to Brawl.