Currently, XTwin is running Dabr r408, which was released on May 30th, 2011.
Most people will use it because even Dabr (in addition to Twitter itself) is blocked at work, but I think a lot of people prefer XTwin because it has better themes.
XTWin is very secure - your Twitter login details are stored in an encrypted cookie on your machine, and never stored on the website, unless you decide to use that feature.
Honestly, nothing. I just wanted a short domain (3 letters + a two letter TLD), so I started looking for names starting with "tw". Pretty much every combination starting from "tw" was taken, so I just added an X before it and stuck with it.
I wanted a short domain because I wanted to integrate a URL shortener with xtw.in, but I got too lazy and never made that. Maybe sometime in the future...
If you have any comments, suggestions or questions then feel free to get in touch.