With XCode 4.1, Apple ships a nice tool to simulate different Network Situations (Modem, 3G (good, lossy), different Wifi Strengths): Network Link Conditioner. You can find it in your Utilities.
Note that as of Xcode 4.4 it is not distributed by default anymore. You can easily install it though by opening Xcode, clicking on the
Xcode Menu -> Open Developer Tool -> More Developer Tools and then download the Hardware IO Tools for Xcode.
Heck, you can even go so far as to create your own profiles!