CSS references added like this
public void renderHead( IHeaderResponse response )
will not be taken into account by Internet Explorer (tested with 8 and 9) at when rendering the document.
I stepped though wicket-ajax.js and saw that the function added in Wicket.Head.Contributor.processComment will actually be executed and a new child node will be appended to the head node of the document. Unfortunately it seems IE disregards conditional comments added in this way.
I encountered this using Wicket 1.5.4.
I'll upload a quickstart.