All iOS developers must know about this handy-dandy tool by Mike Schrag:

speedlimit is a Leopard [works on Lion too] 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. The new version allows you to restrict the slowdown to only a specific set of hosts.