Relevant web page with lots of good info.http://www.slowtwitch.com/Tech/Implications_of_Rim_Width_2803.html
imho, 23c tyres are the idea aero solution for the wider style rims, and generally anything above or below are not as optimal. But going wider to 25c is ok depending on the road surface requirements.
I also believe that the older narrowers rims are at their ideal aero solution with 20mm tyres. And should probably still be the rim/tyre combo choice for track or even crit racers (as road condition is usually pretty close to perfect or is perfect).
But the 20c fad is being phased out somewhat now, because many many people where already using a 23c tyre on a rim optimised for 20c. And it is a no brainer that the 23c tyre matches up to the 23 wide rim
The new wide rim designs are as aero (or more aero depending on wind conditions) as the old 20c tyre/20mm brake track rim ideals. It will just take time for the whole bike industry to move to it solidly.
Their will still be a niece market for 20mm wide rim and tyres, but it won't be the mainstay within a few years I reckon.
(note '23 wide rim' refers to most wide rim aero designs. Some are wider already by the brake track, and even more wider further from the track. Just trying to keep the info somewhat joined properly)