I, too, am hoping to get Sublime Text 2 to behave more like WriterRoom or iA's Writer.
I failed to use the code snippets provided by guillermooo. (I use ST for CSS, HTML and regular text; I haven't programmed in a decade.) I'd be grateful for any help implementing them.
Since ST's API has a show_at_center(point) function, it seems fairly straightforward (for someone who can write Python) to implement the centering effect. The code would be the python equivalent of: after each keypress, show_at_center(where there cursor is). This might break ST's more advanced features, but if all you're doing is writing text, I think it should work okay.
The fade out effect is more complicated, because iA's Writer doesn't just highlight the line, it highlights the sentence. (See http://www.geekosity.org/wp-content/uploads/2011/06/iA-Writer.png) Implementing this would break compatibility with TextMate's themes, which presumably is undesirable.
A rather complicated plugin might be able to grab the text between the two punctuation points (. ? ! for English) before and after the cursor's position and call that regular text, while making everything else styled as a comment (which tend to be themed so that they fade into the background). Of course, this would stumble on a text like "Did you know? St. Jerome was a saint." (It would think that "St." was a complete sentence.)
I hope that the above is not too muddled. I also was not sure whether I should reply here or post a new topic under the Ideas & Feature Requests section. (I'm not activate on forums that much, except when I love the app!)