Earlier U.S. law enforcement raids on lawyers' communications

The prominent attorney Lynne Stewart had her records raided in 2002 over her work with Omar Abdel-Rahman, the "Blind Sheik" linked to the 1993 World Trade Center bombing and the 1997 Luxor massacre. Her contact with her client was subject to special administrative measures that she may not have honored. Prosecutors believed she was relaying messages between her client and his followers. She was convicted on several charges and sentenced to prison. The case generated some controversy: the National Lawyers Guild was displeased enough to issue a scathing report on her treatment. "Democracy Now!" called her a "political prisoner".

Until the present case, what was the most significant or consequent of these actions in the United States?

I don't know about most significant consequences, but I'll note the idea has been around for at least a century.

United States v Clark (1933) references the limits of attorney-client privilege while dealing with the limits of juror privilege. Long story short, a juror (Clark) was found to have lied about things that would have disqualified her. Her behavior during deliberation of the jury was used as evidence. Clark tried to have this evidence thrown out claiming juror deliberations are privileged. The judges disagreed writing...

We turn to the precedents in the search for an analogy, and the search is not in vain. There is a privilege protecting communications between attorney and client. The privilege takes flight if the relation is abused. A client who consults an attorney for advice that will serve him in the commission of a fraud will have no help from the law. He must let the truth be told.

In establishing this principle, they cite O'Rourke v. Darbishire (1920) (a UK ruling) in which the executor of Sir Joseph's will, Mr. Darbishire, is charged with fraud. Mr. Darbishire was also Sir Joseph's solicitor. O'Rourke sought legal documents between Mr. Darbishire and Sir Joseph accusing Darbishire of fraud.

The case established that mere accusation of fraud isn't enough to dismiss privilege, you must also already have some evidence to support the accusation.

But this conception of the privilege is without support in later rulings. 'It is obvious that it would be absurd to say that the privilege could be got rid of merely by making a charge of fraud.' To drive the privilege away, there must be 'something to give colour to the charge'; there must be 'prima facie evidence that it has some foundation in fact.'

The one people seem to be drawing on a lot (I found two recent articles about it) was the raid on Donald Manno in 2008.^* The better of the two was here on Reuters.

In 2008, Manno’s law office was raided by federal investigators pursuing a racketeering case against a suspected mobster who was a client and friend of the lawyer. The FBI copied Manno’s computer hard drives and carted away evidence. Three years later, when New Jersey prosecutors indicted Manno’s client for fraud, money laundering and extortion, they also charged the lawyer.

While he was ultimately acquitted, Manno spent years defending himself. He predicted a tough road ahead for Cohen.

It appears the acquittal only happened in 2014. That's 6 years from raid to resolution.

There's a lot more good information in that article about the process.

Only a handful of law firm search warrants have been publicly disclosed. But based on that record, it is safe to say that law firm raids do not bode well for lawyers or their clients.

New York defense lawyer Gerald Lefcourt said warrants against lawyers almost always end in charges, given the high bar the government has to meet to obtain them. Lefcourt represented defendants in a 1990s racketeering conspiracy case in which the FBI raided the law firm Lysaght, Lysaght & Kramer. The law firm’s name partners were eventually convicted at trial.

There isn't a lot of info I could find online about that LL&K case. There are official court documents though. It appears the named law firm partners after being raided were tried and convicted under RICO for bribing union officials, election law violations, and wire fraud.

So it looks like this is indeed "rare, but not unheard of". The reporting on the kind of activities Cohen was involved in are pretty much exactly what I listed above.

People with knowledge of the inquiry have told The Washington Post that Cohen is under investigation for possible bank fraud, wire fraud and campaign finance violations.

^{* - Most likely this is because he was found not guilty, so he's quite happy to talk about his victory to anyone who will listen. People found guilty don't appear to be quite so ebullient about it.}