imo designing and prototyping should be a too basic and seminal task to have a dedicated tool... it is just not possible to standardize a creative flow. Any program that put less limit as possible is a must, the rest have to came from the blend of the designer technical and artistic background... any bitmap program can do all the tricks a designer need but nothing can beat in flexibility a vector based program... sadly most are pretty dull (illustrator/freehand, coreldraw...) but Xara prodducts really stand out for the quality of the engine, the speed and the ability to export in flash (pretty standard solution in creating semi functional demos)