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

^A - move to start of buffer
^E - move to end of buffer
^U, Shift-Del - delete (clear) buffer
Shift-Clear or Ctrl-Clear: clear buffer and hide input box (show status)
^W - delete word to left of cursor
Left/Right arrows - move cursor
Backspace - delete the character to the left of the cursor
Ctrl-Del - delete the character under the cursor

Future plans:

Start+S: hide (but do not clear) the input box; show status. The
input box will become visible again on the next keypress.
Shift + Ctrl + Up/Down arrows - history (if we can spare the RAM)
Up/Down arrows - move up/down by one line (40 chars)
^Y, Shift-Insert - paste (^K, ^U, ^W fill a paste buffer; need RAM)
Ctrl-Insert - toggle insert/typeover (does anyone care about this?)
Atari key - insert a ^B (meaning, toggle bold)
^U should be "delete to start of buffer", not delete whole line.
^K - kill (delete) to end of buffer.
Ctrl + Shift + Left/Right - move back/forward one word (space-separated).
Tab - tab completion. For [server], complete channels. For [private],
complete nicks that have PM'ed us or that we have PM'ed. For channels,
complete channel nicks (we'll never have enough RAM to have full lists;
search back through screen memory is how it'll work)

The glyphs for these will appear as inverse letters, but will actually
be the appropriate low ASCII characters:
^I - toggle italic
^L - toggle underline
^S - toggle strikethrough