on the bus home i heard a couple guys discussing their mobile plans with one another. one mentioned he got service with nextel and they "hooked him up with unlimited text messaging and chirps" on his plan. it took me a second to grok what he'd said, but i think i've got it...
chirps - two way radio service mostly found on nextel and boost mobile phones. the service is extremely popular amongst teens, inner city youth and business people. characterized by the slightly irritating chirping noise that lets the user know the person on the other end is done talking or they can start talking.
it's the first time i'd ever heard it referred to as chirps, but it sort of sums the service up pretty well. so remember, you heard it here first. from a white guy.
updata: according to andre, there's a whole flock of nextel billboards up and down the 99 [see: fresno] that are promoting the term "chirp".