I just went through an interesting sugyah from teh 4th perek in Kiddushin. The gemara says that when Moshiach comes Eliyahu will be mitaheir the mamzeirim. The Ramban learns that even a mamzer vadai will be muttar. The Ran asks that how can it be that the issur of marrying a mamzer will be battul. Rather the gemara means that any mamzer who is mixed in with klal yisroel and we don't know that he is a mamzer will now be muttar. Eliyahu will not reveal the fact that he is a mamzer.
The Ran says that this has a nafka mina b'zman hazeh. Even if someone knows that a person is a mamzer, he can't reveal it. Just like Eliyahu knows who is a mazer and won't reveal it, so too we can't reveal who is a mamzer. The Rema in Even HaEzer (Siman 2) brings the Ran l'halacha.
There is a machlokes between the Chelkas Michokeik and Beis Shmuel (Even HaEzer Siman 2) whether this halacha applies to 2 eidim. The Chelkas Michokeik says the Ran is only referring to a situation where one eid knows someone is a mazeir, or where there is a rumor about a person then we don't investigate. But if 2 eidim know they have to tell beis din. The Beis Shmuel holds even 2 eidim can't say anything.
The Chazon Ish (Even HaEzer Siman 1) seems to pasken like the Chelkas Michokeik.