I had it in my waters with my first baby who had to be quickly vac extracted because of it (it was fairly dark). I always thought it was symptomatic of distress so i'd say it was dangerous.... however I found this article from the Purebirth site:

Meconium Isn't the Problem; Induction Is - by Gail Hart of midwiferyeducation


Meconium (mec) is not a problem- unless mec is a symptom of severe distress. And then the problem is the distress- not the mec. With good fetal heart tone and a normal labor, even thick mec is rarely a problem.

Meconium is more common in labors which are induced, by any and all means of induction, and it's debatable whether mec is even more common in post-dates labors since induction is often a confounding factor. I recently came across a study (as follows) that looks at the liklihood of whether heavy mec is actually a risk for meconium aspiration. It's older, but large- it looks at almost a thousand babies with "thick or moderate meconium". All sorts of things were found to contribute to the rate of mec; but only a couple babies out of a hundred actually developed meconium aspiration syndrome- MAS- even though the entire group had mec (39 out of 937).
Induction was a strong link to both meconium waters and to meconium aspiration syndrome- but (and this surprised everyone) POSTDATES was not found to be a factor in the babies who developed MAS: MAS was distributed equally among all gestation groups. MAS was correlated with thick mec primarily when there were other risk factors present- need for resucitation, poor heart tones, or ceserean delivery.
Induction of labor was the strongest association with MAS. We know that we see more mec in induced babies, and we know we see more MAS in induced babies. A logical guess may be that we see more mec in post-dates babies simply because post-dates babies are far more likely to be induced than are 40 week babies.
This study confirms what most of us have seen: that meconium is "rarely a problem"- even when it is thick.

No Benefit Seen With Suctioning During Birth of Meconium-Stained Neonates SAN FRANCISCO (Reuters Health) Feb 11, 2003

Suctioning during delivery of infants who present with meconium staining apparently does not prevent meconium aspiration syndrome. These findings, presented at the meeting of the Society for Maternal-Fetal Medicine, contradict current practice guidelines.

Lead study author Dr. Edgardo Szyld, of the Hospital Diego Paroissien in Buenos Aires, Argentina, believes that "we should consider revising the current recommendations" of suctioning these infants during delivery.

A total of 2514 infants with meconium-stained amniotic fluid were randomized to oro- and nasopharynx suctioning or to no suctioning just before delivery of the shoulders. Of those infants suctioned, 3.5% developed meconium aspiration syndrome (MAS), as did 3.6% of those not suctioned. Five newborns died in the suctioned group, and three in the group not suctioned.

No differences between the two groups were observed in the frequency of thick meconium, C-sections or need for resuscitation.

A single study back in the 1970s was the foundation for the recommendation of suctioning when meconium staining is evident, Dr. Szyld said. Recommendations to suction, set forth by the American Academy of Pediatrics and the American College of Obstetricians and Gynecologists (ACOG) makes the practice "widespread--and it's done around the world."

However, he said, the current study shows that suctioning before the shoulders are delivered does not prevent meconium aspiration or its complications.

"The data presented by Dr. Szyld's team provides convincing evidence that suctioning probably does not" alter outcomes, Dr. Laura E. Riley, chair of ACOG's Committee on Obstetric Practice, told Reuters Health.

"Because suctioning has been beaten into clinicians for so many years, I'm not sure the current findings are really going to change clinical practice," Dr. Riley said. "Still, I think the findings may provide some reassurance to clinicians that when meconium aspiration syndrome occurs it probably didn't have anything to do with how adequately the infant was suctioned."

While Dr. Riley believes that the researchers succeeded in showing that suctioning is probably unnecessary, she said they didn't address "whether suctioning may actually have harmful effects, such as causing facial trauma."