This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
For me it means you should use it for single attributes, not collections. Howewer, the annotation api doesnt deny it, and with eclipselink, and hibernate it works for me.
but in the xml api, which is better-defined, its not allowed.
So I think you should not use it,
the generic type, or the targetClass attribute of @ElementCollection is enough.