Regarding mobile phones - our charger wouldn't work at all in the US, obviously because of the voltage issue. This was years ago though, so it may well be the case that newer ones are fine. We just bought an ordinary Nokia charger in Walmart for about $7, and we always take that now - the plug pins fold flat, and it is as light as anything.
I also bought a hairdryer years ago in the US, and always take this, but the travel ones here with dual voltage are just as good. If they don't have dual voltage, they just run really slowly and take forever to dry your hair!