Y'know, I really get annoyed at the non-accessable search function on these forums sometimes...
Google got me the following.. but I know there was a fairly recent one that got in depth on the subject that is not showing up.
Spells - Seeing/attacking out of Darkness
http://www.enworld.org/showthread.php?t=127549
Darkness better than Deeper Darkness
http://www.enworld.org/archive/index.php/t-76858.html
Real Magical Darkness
http://www.enworld.org/showthread.php?t=123822
Short version as I recall it:
Per RAW, Darkness turns the current lighting condition to that of 'shadowy illumination' regardless of the current lighting condition. Therefore providing the odd circumstance of casting the spell in pitch black conditions and being able to enlighten the area with a 'darkness' spell.
Consensus reached was to HR that 3.5 Darkness caps the areas lighting condition at 'shadowy illumination' on the bright end of the scale. This means casting the spell in darker conditions has no visible effect while maintaining the spells primary function.
Kind of like turning a dimmer switch down.
Normal interaction with Light spells, so having a Darkness spell in a dark area will still have a benefit.
My input, and the way I handle it, is that a scale exists for lighting conditions in an area. Darknss spells reduce the condition 1 or more steps, light spells increase the condition 1 or more steps. Fiarly easy to do using a point based spell system
o- Blinding Sunlight
o- Bright Daylight
o- Cloudy
o- Shadowy Illumination
o- Darkness
o- Deeper Darkness
YMMV, etc..