The 2nd U.S. Circuit Court of Appeals ruled that as long as the recording is going to be used for legitimate purposes, then the recording itself is not a violation of the Wiretap Act. “The defendant must have the intent to use the illicit recording to commit a tort of crime beyond the act of recording itself," the court said (.PDF). The appeals court noted that several of its "sister courts" had ruled similarly.
Simply stated, if you were planning to use the recording for blackmail, that would not be legal. If instead, as in this case, the recording was to be used for proof in a dispute, that may in fact be legal.
In this case, Caro v. Weintraub, a civil lawsuit was filed over a secret audio recording made by a son in relation to his mother's will. Days before she died, the family gathered to discuss handling of her estate. The recording took place between the son, mother, stepfather and others.
The son used a 99-cent iPhone app, Recorder. When the woman died without leaving a will, the son used the recording to challenge his stepfather's claims with regards to the estate.
His stepfather filed a lawsuit in federal court in Connecticut. He claimed his stepson violated the Wiretap Act when he recorded the kitchen conversation. The case was dismissed, but the stepfather appealed. Now that appeal has been dismissed, as well.
Interestingly, the latest version of Recorder has a way to effectively record outgoing calls. There are other apps that do this, but Recorder actually works with the phone company and records a high-quality version of the conversation. There's a per-minute charge for this, however.