not sure if this belongs in movement or general fitness, so mods feel free to move this as you see fit. 

i'm so happy i finally got this down today!  i knew i had the strength, but there was something lacking in the technique..
http://aphtyr.avara.net/tmp/08jul/climbs.mov (13.4 MB, sorry my host is a bit slow)

if you've been struggling to muscle-up on a bar, i hope the following will help you achieve it:
0. pre-requisite: climb-up onto a wall
1. wall climb-up with less assistance from feet (1 foot, off toes, etc.)
2. ledge/wall climb-up with no feet
3. ledge climb-up onto progressively narrower (and/or rounder) surface (you might have to search around for places, but this is what did it for me.  a climb-up onto a square-edged bar about 1.5 inches wide taught my body all it needed to know to muscle-up on a bar)
4. muscle-up on a round bar!  (just pretend it's square-edged.  physically, it is the exact same move!)

there's always the beastskills tutorial too.  and if you can't do the pre-req, i would recommend clapping push-ups or any other type of explosive movement.  i honestly can't remember how i learned to climb-up, but i know it used to be another "impossibility" for me. 

my next steps are to develop ambidexterity and more cleanliness/smoothness in the move