RonPK - 1:05 pm on Oct 13, 2011 (gmt 0)
Erm... I'm pretty sure they've been doing this for at least a month or so.
Also, on twitter.com and in clients such as Tweetdeck, the original URL is displayed as anchor, with the t.co URL in the href.
And yes, URLs from bit.ly and tinyurl.com are also wrapped. Even URLs from is.gd, which are shorter than the t.co URL.