I second Christina's suggestion: my favorite is Hootsuite. It has the best set of features for managing multiple social media accounts and scheduling automated updates. They launched a pretty robust analytics system earlier this year, too, that will allow you to really understand the impact of your efforts.
It works with Facebook, Twitter, LinkedIn, Ping.fm, Foursquare and more. You can give it a whirl with a free account before you decide if you or your client will want to pay for the Pro service.
All that being said, it's been a few months since I last did some research on this so I'm really curious to see what other people have to say.
Good luck, and let us know what you choose.