Japanese Ratoc Systems Corp. has a new lineup of wireless audio products called the "REX-Link" series, and some of them are even specifically designed to fit your 3/4G iPod or iPod mini.
Original link from Gizmodo
There are four products in the series—two "receivers" and two "transmitters." Receivers come in the form of the "CR-RX01" with optical and analog audio outputs or the "REX-WHP1" headphones. For your transmitter, you have two options: the "CR-TXB01" USB transmitter, or "CR-TXB02" USB/analog transmitter (which also attaches to the back of 3/4G/mini iPods). These four products are matched to give three available packages: one with CR-TXB02 analog/USB transmitter and CR-RX01 receiver, one with CR-TBX01 USB transmitter and REX-WHP1 headphones, and one with CR-TXB02 analog/USB transmitter and REX-WHP1 headphones.