Hypersmurf
Moderatarrrrh...
Gaining evasion from a ring would not count as an "innate evasion ability".
No, I realise that, but it's a sign that the Light Armor restriction is a class feature of Rogues and Monks. Like the Light Armor restriction on a Ranger's Drizzt feats. Like the Medium Armor restriction on a Barbarian's fast movement.
It's not an intrinsic trait of the Evasion ability.
The fact that they specify for Shadowdancers - who are not the Rogues or Monks referenced in the DMG description of the ability - is another point in favour of "if it doesn't say it, it ain't there".
-Hyp.