This is a very complicated subject, What Can You Monitor And What Is Illegal. Monitoring Times had an attorney write an article on the subject. The ECPA was passed in 1986, but another law was passed in 1995 that is much more restrictive. I encourage everyone to read the article and make up your mind. ONly part of the law is aimed at cellular frequencies. Here is one comment from the article:
However, on October 25, 1994, Public Law 103-414 was enacted. This law amended the ECPA to provide equal treatment to cordless telephone conversations as cellular ones. However, it also amended the definition of "readily accessible to the general public" to exclude all "electronic communications." As noted above, electronic communications include virtually all radio communications. And so, as the law now stands, there is virtually no radio communication that is "readily accessible to the general public." In essence, the lawmakers have closed up tight this most useful exception to the general rule.
The federal government has cracked down hard on radio listening. At this point the only legal listening outside the broadcast bands is:
(a) a communication relating to ships, aircraft, vehicles or persons in distress;
(b) a broadcast by any governmental, law enforcement, civil defense, private land mobile or public safety communications system, including police and fire;
(c) transmissions on the amateur bands, citizens band or general mobile radio services as well as any marine or aeronautical communications system;
(d) satellite transmissions of cable programming as long as the transmission is not encrypted, there is no monetary gain by the viewer, and there is no marketing system available (meaning no one is selling the rights to view the programming via satellite).
(e) a radio transmission which is causing interference with any lawfully operating station (including ham radio operators), or is causing interference with any consumer electronic equipment, to the extent necessary to identify the source of the interference.
Now to me it makes it illegal to listen to Nextel as well as cellular. But as I said you should read up and make up your own mind.
Here is the whole article:
http://www.grove-ent.com/LLawbook.html#Comment