It's pure Nathan Barley to use live, but iphone/ipad has a bunch of good synth apps if you aren't scared of softsynths, and (in the case of iphone) you couldn't really get any more portable.
I bought the relatively expensive Korg iMS-20 app on my borrowed from work ipad, it seems really promising tho I'll feel pretty foolish using it live. (but also won't care) (also probably won't dare bring borrowed ipad to a gig anyway) It has good reviews re:authenticity of sound tho i have no basis for comparison, pretty sure it's more than adequate for my purposes anyway. With the presets and sequencer stuff it's all a bit geared up to dance music but the actual synth operates the same as the real thing.
I do use Jasuto on the iphone quite a bit, it's really ideal for 'experimental'/noise purposes; modular synth approach without modeling any specific hardware, kind of inspired by Reactable with nodes that auto or manually connect and can be moved around to change parameters. You can use really long high fi samples/loops, it's possible to make all sorts of sounds ranging from really digital/glitch to analog-sounding, there are various different distortion modules and also analog-simulation distortion on the output instead of digital clipping... Also there are interesting possibilities you wouldn't get with a normal keys and knobs-based synth eg using the mix (or a line in with adaptor) for feedback, using motion of the iphone to control the sounds, record automation of moving the nodes, etc.
Got a bunch of other synth apps but most are for my liking too geared up to keyboard-based playing, or are kind of one-trick ponies especially some of the more 'experimental' ones. Jasuto is the one I've found most genuinely usable.
