Just seen and had brief demo of the Mackie dl1608 iPad controlled digital mixer. Not exactly budget but a fabulous use of an iPad and I would actually say it is good value given it's capabilities.

http://www.mackie.com/products/dlseries/

Totally excessive for OMB setups but this would be perfect for one my bands (5 piece British 60's / 70's drums keys guitar bass vocals) where everyone wants a different monitor mix. Only big issue is the lack of an on/off footswitch socket for the built in global effects!