Will things like electric shavers, hair dryers etc. from UK work
straight from the mains (using the 2 flat pin adapter) in USA? Does
one need a "step down" transformer / voltage converter?

With the current exchange rate save yourself some hassle and weight in your
baggage and buy them when you get there - i got a set of hairclippers from
wal-mart for 10USD - about 6 quid now. Not worth carrying mine there for
that. Although I am long past the need for hairdryers i am sure similar
prices apply.If you are stopping in a hotel in my experience hairdriers are
provided in most of the hotel rooms