I am no longer able to maintain this project. Please feel free to fork and take ownership.

Speedlimit

SpeedLimit is a Leopard preference pane for limiting your network
bandwidth to one of a couple different speeds — 768k DSL, Edge, 3G,
and Dialup. This is really handy for testing your iPhone app under
normal Edge network conditions in the iPhone Simulator.

Instructions

You should just be able to build the xcodeproj and install the
resulting build product .prefPane.

Download

The binary is now available on my github page.