Latest creative drivers (2.8.0004) has enabled DD-DTS hardware decoding on certain X-Fi cards however I was able to enable it on my X-Fi Elite Pro SB055A with Vista 64-bit SP by using this set of hacked dat files : http://vbkesha.prgr.org/2_8_0004(X-Fi_DD_DTS_FIX).rar
So I though I just share my success with you.
EDIT : For vista, service pack must be installed (Thanks VBKesha)
UPDATE: Just bought a set of logitech Z5500 digital speakers and I was able to send SPDIF streams directly to them under vista. So spdif passthough is working too.