The untitled document is closed automatically once you open an existing file (as long as the untitled document isn't modified).

This sounds like a good idea.

Did you try to use the list of recently opened files for this? http://www.textasticapp.com/v4/manual/lessons/How_can_I_show_a_list_of_recently_opened_files_How_can_I_close_the_current_file.html

A trial version of Textastic is now available for download at http://www.textasticapp.com/mac.html

I guess it depends on the keyboard layout. The default shortcuts on a German system are very different from the ones you're describing. I've added it to my todo list.

Safari does use ⌃⇥ on my machine. I'm using Mac OS X 10.8.4 and Safari 6.0.5:

Xcode uses ⌘{ and ⌘}.

I had a look at Ack. One problem I see is that it only searches one line at a time. So you can't for example do a regular expression search that spans several lines.