no suport for id() in xpath

Using the function id() in an XPath doesn't select nodes against all of the backends (it may work in selenium. The best reason I could find for this is summarized in this thread:
http://<email address hidden>/msg02134.html

Basically unless we have something that tells the backend that ID is the ID it won't work. However, this doesn't explain why it doesn't work in HTMLUnit -- they should at least indicate that.

This requires further investigation.

It should however be noted that since all backends support the "id:itemID" predicate this is not a tragic problem at this time.