@Leland - Agreed. TweetPress is doing some interesting things (for free too), but it's probably a bit over the top to mimic EVERY detail of twitter.

Moving along... what about all the other Clone themes?
eg,
* Facebook - http://demos.foxinni.com/
(I think Justin Tadlock even did a fB inspired theme)
* WooThemes - Gazette - ala CNN
* http://www.wpzoom.com/design/15-great-examples-of-cloned-wordpress-themes/

The bottom line appears to be how _closely_ the theme has borrowed from the original. Pablo Picasso was rumored to have said something like, "Good Artists Borrow, Great Artists Steal" - Of course I think someone else actually said it and he simply "borrowed" it :)