Why doesn't returning false from OnClientClick cancel the postback

I have a LinkButton where I use the OnClientClick property to ask the user whether he really wants to perform an action, e.g:

<script>
function confirmDelete() {
  return confirm('Do you really want to delete?');
}
</script>

<asp:LinkButton runat="server" OnClientClick="return confirmDelete()" ... />

This pattern usually works, but on this specific page, it doesn't. No matter whether I click OK or Cancel in the confirm dialog, the postback is executed.

Just for completeness (to answer pst's question): the rendered HTML is OK. E.g. it looks like this:

<a id="ctl00_c1_content_btnDelete" onclick="return confirmDelete();"
 href="javascript:WebForm_DoPostBackWithOptions(new WebForm_PostBackOptions(..))"
... >
  Delete
</a>