I don't know anyone who is using a smart phone to "design"...but I've found it immensely useful for field work to have such a variety of calculators, databases, and measuring tools on hand.
One other great app is the Bosch photo something-or-other. You can capture a picture and then impose dimensions/text directly onto the photo to download or email later.
PE, SE
Eastern United States
"If a builder builds a house for someone, and does not construct it properly, and the house which he built falls in and kills its owner, then that builder shall be put to death!"
~Code of Hammurabi