[copied and edited from the NanoStudio thread, excludes generative apps that are also great, mainly focusing on ground-up production]

hardware:

iPhone 3GS 32gb, Blue Mikey 2 (also provides stereo line in)

recording and arrangement apps:

NanoStudio - 4 synths, drum machine, mixer, sampling, best £9 I've ever spent (works beautifully on iPad at x2 for now)
Looptastic Producer - for improvising and mashing loops from NanoStudio (iPhone, HD version for iPad)
FiRe field recorder (iPhone)
Fourtrack - for adding audio tracks to NS or LP arrangements (iPhone)
Ricepad - early days but seems innovative in it's operation (iPhone)

'instrument' apps:

Thumbjam - expressive and versatile instrument, the best I've used by far (universal)
Jasuto - modular synth madness (universal)
Curtis Heavy - granular destruction (also iPad dedicated version)
Moog Filtatron - as it says on the tin (iPhone)

Audio (loops and full length tracks) can be moved between apps using one of two audio copy/paste standards. Many apps allow audio to be shared via Souncloud, email etc. NanoStudio allows wav export of either sequencer tracks or mixer channels for use in your 'real' studio.

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