Resilience
According to WoWikki, the formula for resilience, as of Patch 4.1, is this:
Percent Damage Reduction = 1 - .99 ^ (Resilience / 79.12785) ... where the ^ means "raised to the power of".
Understanding the formula is not as important as looking at the actual damage reduction that gets produced as resilience goes up. Here are some sample numbers:
| Resilience | Reduction % |
| 500 | 5.25 |
| 1000 | 10.5 |
| 2000 | 21 |
| 3000 | 31.5 |
| 4000 | 42 |
| 8000 | 84 |
Notice that doubling your resilience from 1000 to 2000 doubles the damage reduction. It doubles again if you go from 2000 to 4000. It would double again if you went from 4000 to 8000, however, finding gear, gems, enchantment, etc. to gain 8000 resilience is probably impossible.
You need to think about it like this. "If I have 3000 resilience, I get a 30% damage reduction. I only gain another 10% reduction in going to 4000 resilience, and maybe those slots could be put to better use with other attributes."
This handy online calculator will tell you the damage reduction that you get from your specific amount of resilience: Resilience Calculator for Wow (Cataclysm)