Your users won't always have blazing fast 3G connections when they're using your iPhone app. They might be in a building that impedes signal strength, or on a train, or whatever. Therefore, you'll want to make sure that your app has a great user experience even with low speed connections.

The simplest way to do this is to turn on a speed limiter whilst your developing your app. This way, as you test the app in the simulator, you'll get a feel for how it will be for users on slower connections.

I particularly like Speed Limit.

You can download and install it on OSX, and then setup the sites you want to slow down. When the simulator attempts to connect to those sites, it will be slower.

I leave the Speed Limiter on at all times during development, and set it to it's lowest speed setting - the equivalent of a 48k Dialup connection.