[dragonwriter]

> Do you assign an ETag that represents the current state of all those resources, so you can tell me if any have changed behind my back?

That would be logical, yes.

> Do your storages even give you the option of updating them all together atomically?

Depends on your backend implementation. There's no reason a composite resource needs to support PUT at all, though (that's one of the reasons that you are likely to want the representation of the composite to include URLs for each component.)