Here is a good example of a workflow for prop design. This concept started off with a process called 'Kit bashing'. It is a process that works well on mechanical concepts especially if you want your designs to have an instant familiarity to them, the idea being that someone will look at the concept and be able to instantly work out exactly what it is and what it does. In this case it's a sniper rifle I designed for a Volatile Games title form last year. You basically gather alot of reference for existing weaponry and throw it all into the mix, taking a trigger and grip from one weapon or a particularly cool looking scope or butt from another. You can then paint over fairly roughly to fill i the gaps or add additional pieces and hey-presto, you end up with a cool design that people will hopefully understand instantly without explaination.
From the finished 2D concept I then went on to model the weapon in Maya, importing the original 2D concept as a texture and a guideline. Finally you again use the original 2D concept to planar map onto the model as a texture. If you have lined up your model well with the original concept this works really well and doesn't need alot of paintover once you render out the scene and work over it in photoshop.
What you end up with is a lovelly 3D view of both sides of the weapon which is great for the approval process and for the proper 3D modellers to come in and do their stuff.