The feral WotLK thread didn't seem too interested, so I'm posting this back here. Does the math seem correct to you?
The formulas I believe apply to feral druids at 80 (based on this thread) are as follows:
PreDR dodge: x = .024 * agil_from_gear + .025414 * dodge_rating + .0081326 * defenserating_from_gear
PostDR dodge: x' = (c*x) / (x + c*k) = 1/(.008555 + .972/x)
Actual dodge: base + talents + x' = 7.039 + 10 + x'
Net avoidance (boss):
A = (miss + scorpid + 0.0081326 * def_rating) + (17.039 + x') = (4.4 + 3 + .0081326 * def_rating) + (17.039 + 1/(.00855505 + .972/x))
~= 24.44 + .008133*defr + 1/(.008555 + .972/(0.024*agil + .02541*dodge_rating + .008133*defr))
In particular, let +defense and +agil = 0 and dodge rating = z. Then net avoidance A = 24.44 + 1/(.00855 + .972/(.02541z)),
and A = 24.44 + 1/(.008555 + 38.25/z).
dA/dx = 38.25 / (.008555*z + 38.25)^2
That last function goes from 0.0261 at z=0 to 0.01746 at z=1000, to 0.01248 at z=2000, and 0.007287 at z=4000.
That function represents the return on dodge rating at that level of avoidance (which is proportional to the return on agility). This derivative is nearly linear in the range from 0 to 4000 (visibly not so, but the curvature is light),
Net Mitigation Delta is a separate matter: It's calculated relative to avoidance, (1% dodge at 98% avoidance is worth more than 1% dodge at 25% avoidance). The A_factor is the amount of net mitigation adding one dodge rating to your gear would give you. For example, with no gear on (24.4% avoidance), adding one dodge rating reduces total damage taken over a long fight by 0.0346%.
The function is A' * 100 / (100 - A) .
| Dodge rating | Net Avoidance | dA/dx | A_factor |
| 0 | 24% | 0.0261437908497 | 0.034600040828 |
| 200 | 29% | 0.0239529354539 | 0.0339492518355 |
| 400 | 34% | 0.0220263608907 | 0.033392890557 |
| 600 | 38% | 0.0203232087395 | 0.032922903337 |
| 800 | 42% | 0.0188102218843 | 0.0325327843664 |
| 1000 | 45% | 0.0174601085875 | 0.0322173364858 |
| 1200 | 49% | 0.0162503030586 | 0.0319724898597 |
| 1400 | 52% | 0.0151620163779 | 0.0317951661714 |
| 1600 | 55% | 0.0141795025616 | 0.0316831795826 |
| 1800 | 57% | 0.013289485785 | 0.0316351683612 |
| 2000 | 60% | 0.0124807095543 | 0.0316505531157 |
| 2200 | 62% | 0.0117435790311 | 0.0317295192189 |
| 2400 | 65% | 0.0110698751461 | 0.0318730224188 |
| 2600 | 67% | 0.0104525244932 | 0.0320828179552 |
| 2800 | 69% | 0.00988541290422 | 0.0323615148525 |
| 3000 | 71% | 0.00936323347922 | 0.0327126585668 |
| 3200 | 73% | 0.00888136198092 | 0.0331408469762 |
| 3400 | 74% | 0.0084357541047 | 0.0336518870068 |
| 3600 | 76% | 0.00802286034203 | 0.0342530022563 |
| 3800 | 78% | 0.00763955507654 | 0.0349531061834 |
| 4000 | 79% | 0.00728307725685 | 0.0357631613656 |
| 4200 | 81% | 0.00695098053549 | 0.0366966538702 |
| 4400 | 82% | 0.00664109118615 | 0.0377702243403 |
| 4600 | 83% | 0.00635147244282 | 0.0390045162271 |
| 4800 | 84% | 0.00608039416474 | 0.0404253304338 |
As you can see, returns go from diminishing to increasing somewhere around 60% avoidance, or 2000 dodge rating/2100 agility..
Edit: Updated to use the more recent numbers Whitetooth gave us below.