Rugged mobile phones, which are water, vibration and dust proof are useful for people who work outdoors and appeal to those who like the outdoors look. Current models available in Australia include: Nokia 3720 Classic, Samsung B2700, Sonim XP3 Quest, Telstra Tough T90. The Sonim is at the extreme end of ruggedness, claiming to meet the IP-67 rating ("dust tight", "powerful water jets") and MIL-810F for vibration. The Nokia model looks most like a normal mobile phone, but still claims a IP-54 rating ("dust protected", "splashing water").
However, with a rugged phone you ten to end up with a unit which is more expensive, larger, heavier and has fewer features. As an example the rtugged unit may only be a GSM phone (not 3G). These phones tend to also have smaller screens.
Also keep in mind that the dust and water resistance ratings depend on little rubber plugs covering the USB, earphone and charging socket. If you change your phone every day or transfer from a car kit, theses rubber plugs tend to get broken or lost. Genuine military specification equipment comes with sockets which do not need covers to keep the water tight or with plugs which are firmly tethered by short chains. While marketing for rugged mobile refers to military specifications, real military users tend to use ordinary mobile phones, kept in waterproof cases, rather than ruggedised units.
As an alternative you can add a soft rubber sleeve or hard case to the more popular models of smart phones. As an example the OtterBox Defender Case series for Nokia E71, Blackberry and Apple iPhones. These hard cases are not necessary water proof. Also they are not available for the lower cost phones, but the combination of a higher cost smart phone and case may still be less than a specialised rugged model phone.
Keep in mind the rugged phones are designed for making phone calls, not web access or GPS navigation. Also these phone depend on the terrestrial phone network. If you are going to make calls from out of the way places.