The term "MIDI" is an acronym for "Musical Instrument Digital Interface"
and as such all letters should be capitalized.  An "interface" is not a
physical thing but is a description of an electrical or electronic
communication standard such as "RS232 Interface."  MIDI is a standard
which describes how musical instruments are to electrically communicate
with each other.

However, the ongoing corruption (or advancement) of the English language
has changed the word MIDI.  We now speak of MIDI Organs, MIDI Keyboards,
MIDI Sequencers, etc.  I have even seen the phrase "MIDI musician."  So
at this point it probably doesn't matter much if the letters are
capitalized or not.

Terry Macham¶
Kimberley, BC, Canada