I've made this mistake several times in the past. Never found an elegant way of doing it so I just wrote an extension method:
public static T[] Load<T>(this IDocumentSession session, ValueType[] ids)
{
var typeTagName = session.Advanced.DocumentStore.Conventions.GetTypeTagName(typeof (T));
return session.Load<T>(ids.Select(x => string.Format("{0}/{1}", typeTagName, x)));
}
I think it's something like this, just wrote this out of memory.