The behaviour of the Eater of Worlds has been changed in the 1.0.5 version of Terraria.
I cannot find an explicit reference to the behaviour change in the changelog, but the following line could be interpreted that way:
Changed the way worm enemies spawn.
The wikis don't seem to have been updated yet, but I have observed that you can no longer fight the Eater of Worlds in any biome other than Corruption. If you move away from a corrupted area during the fight, you will see the Eater of Worlds has been slain! message.
If you managed to destroy any of the segments before you received that message, the loot that was dropped will still be there (unless you have saved and exited the game), so you can go back and pick it up. However, if you moved off the corruption without destroying any of the segments, I'm afraid that no loot will have been dropped.
With the 1.0.6 version of Terraria, you should no longer see this message in this scenario, and should instead be notified that the Eater of Worlds has fled.
From the 1.0.6 Changelog:
NPC Bug Fixes
- EoW is no longer "defeated" when he flees.