I previously posted in Gittin, that it would seem that DNA would qualify as a siman muvhak in halacha - http://hearos.blogspot.com/search?q=dna
Subsequently, I found in a sefer called "atzei besamim" (16) from r' mendel senderovic (Milwaukee), who has a very nice discussion about the admissibility of DNA evidence in beis din. The issue begins with the poskim not willing to allow blood testing to be sumbitted to determine paternity in a beis din. He mentions that the tzitz eliezer (13:104) strongly rejects the use of blood testing for paternity, but the proofs that he cites are very weak (bordering on purim torah). The tzitz eliezer also comments that we can't admit evidence that relies on scientific discoveries, since these things are often just theories that are disproved. However, regarding DNA evidence the margin of error is so slim, and there is hardly room for these types of errors, and is as good as a picture. R' Senderovic suggests that perhaps the tzitz eliezer was writing this at a time when this type of testing wasn't as clear, but perhaps he would feel differently nowadays. In conclusion he agrees with the approach of R' Zalman Nechemia Goldberg and R' Elyashiv who consider DNA evidence to be a siman muvhak to be matir an aguna and determine the paternity of a child.
What about a situation which requires actual "eidus", not just a siman muvhak - Can DNA evidence be considered like "eidim" or perhaps even better than "eidim"? Although at first glance, it wouldn't seem at all plausible, there is a proof from our gemara that eidus can sometimes be achieved without eidim. The gemara says that if a group of eidim testify that reuven was killed, even if 100 witnesses would contradict them, the concept of תרי כמאה would not allow us to give malkus to the original group for lying. Yet, בבא הרוג ברגליו, when the victim shows up in beis din, we can determine conclusively that he wasn't killed and give malkus to the original group for lying in court. The maharatz chiyus points out that even if reuven shows up, the original group can claim that it isn't actually him, so that 1000 people claiming that it is reueven, should not help to convince us that the original group deserves malkus - so why does the gemara say that they get malkus? The maharatz chiyus concludes from here based on tosafos in yevamos 87a - דבדבר הידוע לכל העולם לא שייך תרי כמאה, וכיון שהאיש הזה ניכר לכל העולם לא מועיל אמירת העדים שאמרו שנהרג, ודבר זה אין נופל בגדר עדות כמו שלא יועיל אמירת שני עדים על עץ שהוא אבן והכחשת דבר שניכר בחוש, ונכון
Based on this approach, factual evidence such as DNA which is far more conclusive that circumstantial evidence (which is not admissible), with a margin of error close to one in a trillion, should be even stronger than בא הרוג ברגליו - they would both not account for an identical twin (i think). Being that DNA is scientifically considered "fact" as much as בא הרוג ברגליו is considered fact, it should be even stronger than eidim to work not only in aguna cases but even in cases where "eidim" would be necessary.
כמובן, i am only pointing this out as something to think about - ולא למעשה