I have upgraded to wicket-1.4.4 from wicket-1.4.3. I have made sure to recompile all dependent libraries against 1.4.4. Since switching any component referenced from a JAR comes back with an incorrect content-length.
I can resolve this issue by switching back to 1.4.3.
syris:~ victori$ curl -I http://lfab:8480/resources/wicket.contrib.IconElements/vcard.gif
HTTP/1.1 200 OK
Date: Sun, 13 Dec 2009 23:02:11 GMT
Last-Modified: Sun, 13 Dec 2009 23:00:44 GMT
Expires: Mon, 14 Dec 2009 00:02:11 GMT
It should be 619 bytes.