Sure, lots of murder trials lead to guilty verdicts despite a complete lack of physical evidence. In Peterson's case, he was having an affair, he claims to have gone fishing in the area where the body later washed up, and there was no other plausible suspect. The facts of the case conform to a certain type of homicide that is common and well understood. One may argue that the evidence is insufficient, and perhaps it is, but speaking only for myself, I would never join a crusade to free Scott Peterson because I think he probably did it.
Similarly, I think Drew Peterson probably murdered his wife. There is no evidence he did so; it simply fits the mold as the most likely explanation why this woman would disappear without any signs of foul play. But, a case where a female roommate and her boyfriend team up with someone they barely know to commit murder... that doesn't fit any mold. It is unprecedented. And the rare cases that come closest to forming a precedent involve young women who are deeply and profoundly messed up in the head, like Justina Morley:
She had a history of self-mutilation. She tried to kill herself twice. She
started smoking marijuana at 12, then moved on to pills, coke, heroin and,
weirdly, pot laced with embalming fluid. She abused herself by having
indiscriminate promiscuous sex. She was twice admitted to a psychiatric
institution. She'd been diagnosed with depression, for which she was taking
medication at the time of the crime.
But hey, Amanda got a noise ticket...