USBfever has released the Retro Cassette Stereo Mini Speaker for iPhone and iPods. The unit plugs into the bottom of a handheld, and provides stereo output in a compact and lightweight design. The speaker allows quick access to volume controls, and does not require a dedicated battery.
It is sold only in black, but supports all iPhones, as well as most iPod models, including the Touch. The Mini Speaker is available from USBfever.com at a cost of $32.