Regardless of the state, it is almost always illegal to record a conversation to which you are not a party, do not have consent to tape, and could not naturally overhear.
Moreover, twelve states forbid the recording of private conversations without the consent of all parties to that conversation. Those states are California, Connecticut, Florida, Illinois, Maryland, Massachusetts, Michigan, Montana, Nevada, New Hampshire, Pennsylvania and Washington.
The statues in these states are the following:
Cal. Penal Code §§ 631, 632; Conn. Gen. Stat.§ 52-570d; Fla. Stat. Ann. § 934.03; Ill. Rev. Stat. ch.720, para. 5/14-1 to 5/14-6; Md. Code Ann., Cts. & Jud. Proc. § 10-402; Mass. Ann. Laws ch. 272, § 99; Mich. Comp. Laws § 750.539c; Mont. Code Ann. § 45-8-213; Nev. Rev. Stat. Ann. § 200.620, as interpreted in Lane v. Allstate Ins. Co., 969 P.2d 938 (Nev. 1998) (holding that Nevada’s “one party” wiretap statute requires all-party consent); N.H. Rev. Stat. Ann. § 570-A:2;18 Pa. Cons. Stat. Ann. §§ 5703, 5704; Wash. Rev. Code § 9.73.030.
The Reporter’s Committee for Freedom of the press publishes a good summary of the state-by-state statutes. The RCFP Summary is available on line and includes citations to leading cases in each state as well as the statutory civil and criminal penalties for violations. (Note: This guide and summary is written for Journalists, and does not necessarily address issues such as recording family conversations or other non-journalistic uses of recording.)
It is still unclear whether courts will hold that the federal protection preempts the state law. Bartnicki v. Vopper, 121 S. Ct. 1753 (2001). This issue will be discussed in a separate post, but the safer course would be to act as if the laws of the stricter state will apply.
As in all cases, the application of the law to a specific fact pattern is something you should review with experienced legal counsel. Not only may there be facts in your particular situation that give rise to a different answer or advice, but the law is (on its own) an ever-evolving creature that not only can vary from jurisdiction to jurisdiction, but also as courts and legislatures interpret or change existing laws in the course of their day-to-day discharge of their duties.