This is kind of a weird mix of UNIX/Emacs/bash and traditional Atari.

Cursor movement:

Left/Right arrows - move cursor left/right
Up arrow - if the inputbox has anything in it, move up 1 line,
   aka 40 characters, or to the start if the text is less than
   40 characters.
Up arrow *in an empty inputbox* - bring up last entered command.
   Can coexist with regular use of Up for movement.
Down arrow - moves down 1 line, aka 40 characters. If this would
   move past the end of the text, moves to the end.
Ctrl-A - move to start of buffer
Ctrl-E - move to end of buffer
Ctrl-F or ctrl-shift-Up - move right by one word.
Ctrl-B or ctrl-shift-Down - move left by one word.

Deleting text:

Backspace - delete the character to the left of the cursor
Shift-Del - delete (clear) buffer
Shift-Clear or Ctrl-Clear: clear buffer and hide input box (show status)
Ctrl-Del or Ctrl-X - delete the character under the cursor
Ctrl-U - delete to start of buffer.
Ctrl-W - delete word to left of cursor
Ctrl-U - delete to start of buffer
Ctrl-K - kill (delete) to end of buffer.

Formatting:

Ctrl-I - toggle italic (prints as down-arrow)
Ctrl-shift-U - toggle underline (prints as right-arrow)
Atari/inverse key - toggle bold (prints as inverse B)

Other:

Tab - Nick completion (only at the start of an empty input box!)
Ctrl-Insert - toggle insert/typeover


Future plans:

^Y, Shift-Insert - paste (^K, ^U, ^W fill a paste buffer; need RAM)
Shift-Return: Maybe... send buffer but do not clear it.

^S - toggle strikethrough