Rockbox

The latest (18th August 2006) manual has the following key sequences :

Minus/Plus
Moves the arrow to/from the filename
and changes between the character bar
and BACKSPACE, DELETE, ACCEPT and ABORT.

Play/Stop
Varies (see below)

Menu
Shift. When the character selection bar is selected
this changes between upper case, lower case,
and accented letters.

Selected option

Play/Stop function

filename
Moves the cursor left (Stop)
or right (Play) within the filename

character bar
Moves the character bar to the next (Play)
or previous (Stop) character.

BACKSPACE
Play deletes the character before
the current cursor position

DELETE
Playdeletes the character at the
current cursor position

ACCEPT
Playexits the virtual keyboard and
saves any changes

ABORT
Playexits the virtual keyboard and
discards any changes

How it actually seems to work is:
On button : Moves focus from the filename to the character bar
Menu button : Shifts between Capitals, Small Letters, and Numbers/Other Characters
-,+ : Moves focus left,right within character bar or filename
Play : In character bar - quick keypress inserts current character in front of currently selected character of filename
In filename - Backspace (deletes previous character)
Long keypress in either character bar or filename - save config to selected filename
Stop : Acts as cancel and exit at all times

I checked this using CVS for 18th Aug 2006 as well as Rockbox 2.5, so it must have been this way for some time.