Monster Manuals have never dealt with the classic “mummy’s curse”. Monster Manual mummies give you “mummy rot”, a disease which has slightly different effects across the editions, but it’s always just a workplace hazard for mummy killers. When you come out of the temple, you head to the cleric to get your disease removed, and that’s the end of it.
The mummy’s curse superstition seems to have something to do with a lingering doom that hangs over the heads of those who disturb mummies,and is something like, “100 years after the tomb was opened, NOT ONE of the excavators remained alive!” Inasmuch as it makes any sense at all, it’s about death under mysterious circumstances, some time after the adventure, of seemingly healthy people. DON’T VIOLATE MUMMY TOMBS, or you’ll get approximately the same kind of bad luck you get by breaking a chain letter, or not being on that one flight.
Could we actually come up with game rules for the mummy’s curse? The 4e Unearthed Arcana “Curses!” article might be a place to start. What I’d like to see, though, is official rules text like
Any PC who disturbs the sarcophagus is subject to THE MUMMY’S CURSE. THE MUMMY’S CURSE cannot be removed by Remove Curse, Remove Disease or any other means short of Limited Wish or Wish. THE MUMMY’S CURSE has no immediate effects. However, the PC should note it on his or her character sheet.
The actual effects of the Mummy’s Curse should be SO HORRIBLE that, lest PCs discover countermeasures, they are NEVER PUBLISHED IN ANY BOOK. Heck, maybe the effects of the curse are determined retroactively. Like, when the PCs are finally eaten by gnolls, the DM leans over and taps the character sheet. “MUMMY’S CURSE,” he says.