The first thought is using an amp with two inputs (one for iPhone and one for your guitar/bass) and headphone socket. (It is what I use.)
Or using a (cheap) mixer that can function in the same way.
I think it unlikely, but have not tried it, using iRig;
http://www.ikmultimedia.com/irig/features/
and multitasking (iOS4) and the iOS simultaneously being able to output audio from iRig software and iReal book app, Player audio (I am not sure it will do this.)
Perhaps some with this combination could try it for us and report back?
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