If you haven’t already setup Ping.fm then you may want to checkout my guide.
Take note of your Mobile Key,
Start Tweetie on your iPhone and then click <Accounts>,
Now click <Edit>,
If you have multiple Twitter accounts select the one you want to integrate Ping.fm with, otherwise just select your Twitter account,
On the Edit Account screen select Advanced,
Enter your Mobile Key obtained from Ping.fm earlier, then click <Done>,
Now click on <OK>,
When you send out a Tweet using Tweetie, all your social networks added to Ping.fm will also update.