Here's what bugging me. Everytime a Defiler uses his magic, he turns the ground he's standing on infertile. So does that happen instantaneously, i.e. does the caster have to move out of the defiled zone to be able to cast a spell again? Or does the affected area merely grow, the two radii added together?
Casting Multiple Spells from the Same Location:
If a defiler casts more than one spell from the same location, the radius of destroyed vegetation expands around him. Consult the Defiler Magical Destruction Table for the highest level spell cast from that location, then add one yard for every other spell cast. (Spells equal to the highest level spell are treated as additional spells).
For example, the defiler Grifyan casts a lightning bolt, a 3rd-level spell, while in the scrub plains. The area of ash around him will be 4 yards. In the next round, he casts a magic missile spell, expanding the radius of ash by 1 yard, bringing the total of burnt earth to 5 yards. In the third round, Grifyan decides to unleash an advanced illusion, a 5th-level spell. Since this is the highest-level spell cast from this location, the area of ash is recalculated; 5 yards for the 5th-level spell, plus 1 yard each for the two lower-level spells cast, for a total radius of ash of 7 yards.
You only need to adapt it a little to work with 3.5 rules...
And I think they defiled EITHER at memorization (off stage spell power gathering) OR when they cast their spells (on stage spell power gathering).. at least as far as I recall...
Actually, defile when preparing only existed in DS2. In DS3, you defile when you cast. Period.
rusty memory, and I have yet to actually play more than a handful of 3e games, none of which are darksun 
Thanks, that's the kind of ruling I was looking for, Bruno. I'll show it to my DM. I think we'll go for radius = sum (spell levels)*5, so casting two lightning bolts nets you a (3+3)*5=30 feet radius of ash. Cuts down on recalculating when someone casts a higher level spell than he started the defiling with.
It might be a good idea to put some sort of limit on how far a defiler can draw energy from. Would suggest 50ft-100ft +5ft-10ft per caster level, for ease of use you could write this on the character sheet under the heading Draw Radius:.
Also, two things about how you calculate defiler radius.
1st, you really need to figure the type of terrain into your calculations because the amount of life around you is essentially how much energy is available in a given area. Using the method you have mentioned. I would suggest that you set a per level variable radius based on terrain type
Dense Forest/Jungle/Swamp/Water Gardens 1ft
Grassland/Crop Fields/Gardens 2-3ft
Scrub Plains/Cactus Gardens 5ft
Rocky Badlands/Stoney Barrens/Sandy Wastes/Mountains 7-8ft
Salt Flats/Boulder Fields/Silt Sea 10ft
Second, the reason the radius was calculated the way it was in 2e is because the amount of area in a 5ft radius is only a fraction of the amount of area in a 10' radius. Every time you double the radius you effectively quadruple the total area affected. So subtracting the original radius you get 3 times as much area affected on the second casting of the same spell as if you just add radiuses together. You can either ignore this for simplicity or you can insert a very approximate rule like the one that they used in 2e. Or, here are some other suggestions:
1. If you are using a battlemap you base your calculations on how many squares or hexes would be effected instead of actual radius. An just build out in a approximately circular fashion
2. You can center the radius for each spell just outside of the last effected area.
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