In current Nokia WAP gateways the WML content is encoded by the gateway while the XHTML content just goes through in text format. First XHTML enabled terminals will use WAP/WSP stack for the transport i.e. XHTML is delivered to the terminal using WAP/WSP stack. So there are WAP gateways available e.g. from Nokia which support delivery of XHTML content.
In the future WAP NG will use TCP/IP stack and an update is needed to the WAP gateway to support TCP/IP.