Just read about Tweetmarks great service. I cannot believe no one has done this before now.
My biggest gripe about Twitter right now is staying in sync between my iPad and iPhone. There are only 2 apps that offer a syncing service right now, Echofon and Tweetings. I use Echofon because it just works better than Tweetings and doesn't try to cram too much into the app.
I prefer different apps though on my iPhone and iPad. For my iPad I just like the official Twitter app. It seems to have the best interface and mix of features. For my iPhone I like switching and trying all different apps. If the developers will add this to their apps, I can run separate apps on the 2 devices and keep in sync on my read tweets. I am hoping that tweetmarks becomes a standard among developers and we see it added to all apps similar to Twitlonger or other services that make the whole experience better.