I believe most of the major exchanges have mobile apps - I can't recommend any particular app since I've never traded on mobile (nor do I have an iPhone):

Bitfinex - https://itunes.apple.com/us/app/bitfinex/id1079033582
Bitstamp - https://itunes.apple.com/us/app/bitstamp/id996483140
Kraken - https://itunes.apple.com/us/app/kraken-bitcoin-exchange/id914671502