It's a controversial subject. What's not in debate is that more aerodynamic spokes improve things (though in degrees subtle enough to only matter for serious time trial competitors); what is in debate is what constitutes the most aero spoke.
There are lots of different condition-specific tests, whitepapers, wind-tunnel experiments and other articles which make arguments in different directions. Some tests favor oval or teardrop spokes; some favor fat, simple blades (like the Ksyrium spoke). Others argue that cross-wind action makes the basic round spoke most effective overall.
Along the same subtle lines, different spoke designs effect the overall mechanics and physical properties of the wheel -- how stiff are the spokes? Stiff in what direction? Rotationally? Torsionally? Does their shape lend vertical compliance? How does that effect rolling speed? It's all complex and, like I've stressed, far too subtle for most of us to worry about.
Just buy good wheels, says I.