It seems that is is not possible to access a property of a class that implements the Collection interface. For example: <jstl:out value="${myCollection.property}"/> results in the tag complaining about not being able to convert a string to an int. It would be usefull if the tag checked whether or not such a property actually exists before complaining.
This behavior is according to spec (see section A.3.4). If you would like to get the Expert Group's attention on this issue, please submit a comment to jsr-152-comments@jcp.org (the EL is now owned by the JSP expert group, JSR-152).