Damage to your device, (even if through ordinary use) can limit or lessen resistance to dust or moisture.

ENGADGET: Sony ships Xperia advance to the US, offers unlocked ruggedness for $300 or less

Despite your device's resistance qualities, you should avoid exposing it to environments with excessive dust or moisture.

ENGADGET: Sony ships Xperia advance to the US, offers unlocked ruggedness for $300 or less