Îles flottantes vs. oeufs à la neige
The short answer appears to be: îles flottantes and oeufs à la neige are two different names for one dish.

First of all, you can find some recipes, in English and in French, using the names interchangeably:
"Floating Islands (Oeufs a la Neige)", Saveur
"Recette de Ile flottante, le classique...POUR LES OEUFS EN NEIGE", L'Atelier des Chef

So clearly, to some people, they are the same thing.

However, in reading through recipes I noticed something that may suggest some difference: in recipes for oeufs à la neige, the meringues always seem to be poached in hot milk (BBC, New York Times, Le Journal des Femmes, L'atelier des Chefs), while îles flottantes meringues are sometimes poached (Saveur, BBC, New York Times), and sometimes baked (Food Network, L'atelier des Chefs). Those poached meringues are supposed to be made into an egg shape (thus, "oeufs") - although more often the egg-shape is only hinted at (e.g. "oval-shaped" rather than "ovum-shaped"), but the baked meringues are generally not formed into any particular shape (unless a dollop is a shape).

I don't think this difference is that significant. Mostly because it's only occasionally a difference. It's possible that one dish was originally poached and one baked, but it seems to me that these baked meringues could probably be classified as "not totally traditional."