TB Bittrex is a bare bones but decent iPhone app, I actually really like it
HitBTC has a mobile web version, it's not great but it works
Poloniex has an Iphone app as well, looks good but I don't trade with them (yet)
Kraken has an iPhone app but it's not good
Tabtrader is one to keep an eye on, supposedly you can trade with it on android, and will be able to add api keys on the iOS version in the future.