For a long time I used Echofon as my main twitter app.  It was ugly as sin but it would sync my unread status between my iPhone and my mac.  Eventually I fell in love with Tweetbot too much to keep using Echofon.  I always missed the syncing though and I usually avoided using Twitter on my mac because I hated having to scroll to find where I left off, not to mention it telling me all of my direct messages were unread.  Thankfully a developer named Manton Reece created the Tweetmarker service.  This is an open service that can be implemented by any Twitter app and will sync your timeline position between multiple apps.  Now, when I open Twitterific on my mac it automatically scrolls the last tweet I read in Tweetbot on my iPhone.  Simply put, I love it!