OK, I checked some sources:
New York Times Style Manual says: “Sometimes whoever or whomever will occur, confusingly, in a clause that is part of a larger sentence. In that case, disregard the overall sentence and choose the pronoun according to its function inside the clause: Give the book to whoever answers the door. (He* or she answers.) Hand the package to whomever you see first. (You see her or him.)"
Bernstein’s The Careful Writer says: “The most common misuse of whom for who occurs in relative clauses in which another verb diverts attention from the verb that governs or is governed by the pronoun.” His example of a misuse is: “One purge victim whom the President apparently believed was innocent was Amelito.” This sentence is wrong because the President believed he – therefore, who – was innocent.
In response to S1969, yes, “who” is nominative case. But here it’s the subject of a clause: “whoever is waiting for it.” And because it’s the subject of that clause, we use the nominative case – who, not whom.
I’ve wasted 30 years of my life correcting other people’s writing for pay, and a lot of that was grammar. Show me a publicly available, professional editor who teaches “Give it to whomever is waiting for it,” and I’ll buy you a saddle.