It's harder to do limited roll outs, but not impossible. It's feasible to have all the code in place and only trigger it based on a server response, for instance. That's basically how they handle the web app.
The same way that testing apps with Easter eggs works. You have to tell Apple that the feature exists, and let them test it. If you don't, and they find out after approval, you run the risk of getting your app pulled.
Would Apple really pull the official Twitter app? It seems highly unlikely, because Twitter has leverage over Apple's users. People won't switch to some other short message sharing service, they'll switch to a non-Apple phone.
