Constricting the Tubes
This will be old news to some, but if you’ve upgraded to Lion and installed the latest version of Apple’s Developer Tools, go take a look in your Utilities folder. There you’ll find a new folder containing a preference panel called Network Link Conditioner. Just click to install it, and you’ve got a handy way to simulate various bandwidth conditions (particularly those of the paltry variety). NLC temporarily throttles your machine’s Internet connection system-wide, so it works just as well for testing the performance of your websites as it does for the proprietary iOS apps it was intended for.

Thanks for the tip.
For people stumbling upon this and not with Lion yet, a great utility is Charles proxy which does have a throttle feature + many other useful features