As noted in last night's tip, recording a conversation without another party's consent is legal in some states but not others. This map, available here, shows states (shaded in dark blue) where the other party's consent is needed as those (in light blue) where it is not.
So while in a great plurality of states you can record a phone call without the other person's consent, in four of the six biggest states (California, Florida, Illinois and Pennsylvania) the second party's consent is required.
A 2012 decision by the 7th Circuit, raised questions about the constitutionality of Illinois' eavesdropping statute. See, ACLU of Illinois v. Alvarez, 679 F.3d 583 (7th Cir. 2012). This statute requires all parties involved in a conversation to consent to it be recorded. However this case concerned a recording made by the ACLU of police officers performing their official duties in public. The Illinois statute makes it a felony to record a conversation without the consent of all parties, even if the conversation is not intended to be private. Massachusetts also has a statute requiring the consent of all parties, regardless of whether or not the the conversation is private.
In California, Florida, and Pennsylvania public conversations can be recorded without consent. While Illinois, Massachusetts and Pennsylvania make an illegal recording a felony, in Florida one can only be jailed for less than one year or fined $1000, and in California an offender can only be jailed for less than one year or fined $2500.
Federal law only requires the consent of a single party. See 18 U.S.C. § 2511.