Via Instapundit, there is a new filing by the prosecutors, suggesting that in one instance, an investigator gave a cabbie $60, knowing that some of it would be handed back the the passenger, who was a witness in the matter. Story here. I recommend not jumping to conclusions.
The allegations do highlight the danger of mixing cash and witnesses. In some jurisdictions, a lawyer may pay a witness a reasonable amount of money for the witness's time, but never for the contents of testimony or for helping to obtain a particular result at trial. Even when payments are lawful, they are always subject to criticism by the opposing party and are fair game on cross-examination or other impeachment for bias.
Readers may remember this matter as the one where prosecutors served a very broad subpoena on the law school clinic, seeking even the students' grades for their clinical work.