Chris,
I have QT 10 on two Macs (one Snow Leopard, one Lion), both with GarageBand
and iTunes installed; when I tried to play a .MID in QT, it had to go
download QT _7_ in order to play.
I checked with Apple, and they confirm there is no MIDI support in recent
versions of QT, so that's why if you try to play an SMF in QT it won't play.
Neither of them appears to have installed a virtual MIDI device, though I
don't have a ton of experience with MIDI devices on Mac, so I may be
misspeaking there. GarageBand appears to directly handle their software
instruments.
CoreMIDI (which comes with every Mac) includes a "DLSMusic Device" audio
unit (with GS/GM sound bank), so all you need to play a MIDI file is an
audio unit host app... GarageBand is one, as it just about every other MIDI
music app available for the Mac.
So, a GM synth engine does come with both OSX and Windows.
- TW