For what it's worth, changing the extension doesn't seem to work for me. A simple C program (http://hastebin.com/36QWBlYlML) gets detected as TeX. Trying to change the http://hastebin.com/36QWBlYlML.tex to http://hastebin.com/36QWBlYlML.c just redirects me back to the .tex URL.
Also, while I am entering a file, the edit box is only two lines long, and I can't see the handle to resize it. This is Safari 5.1.1 on Snow Leopard.
Looks nice!