Magic Missile does not hit the target, and so does not trigger on hit effects
Rules Compendium p214 defines attacks and tells us to "Compare the attack's roll to the target's defense. ... If the result is equal to the specified defense or higher, the attack hits the target", and on p215 notes that some powers do not require attack roles. "Such powers automatically deal damage, impose conditions, or harm enemies in some other way."
Rules Compendium p222 defines damage, and tells us two things: "Each attack specifies how much damage it deals, if any, and under what circumstances: on a hit, on a miss, or automatically.
Rules Compendium p311 defines effect (and unfortunately there is no definition that I can find of "hit"), where it tells us: "In an attack power, the effects of such an entry [the effect line] are not contingent on a hit or a miss.
Magic Missile deals damage as via the effect line of the power, and never requires the caster make an attack roll, meaning that the p214 conditions for a hit are never met. The wording on p215 could use the word hit, but does not, indicating to me that the power damages without hitting. The wording on p222 and p311 both allow for an attack damaging without hitting, and thus do not introduce contradictions.
In summary, it would appear to me that Magic Missile does not hit the target, and so does not trigger on hit effects. Likewise it does not miss the target, and so does not trigger on miss effects either.