The Pod Express works well with Loopy Pro running on an iPad via the USB interface. Presumably an iPhone would work as well. Other looper software would be fine as well.
Pod Express <--USB--> iPad (running Loopy Pro)
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Headphone/Amp
You select the RECORD input and JAM output on the iPad to record the modelled sound and have the Express play it unaltered. Turn the monitoring of the input off in Loopy Pro (or else the sound will double). To put the looper in the front (to audition sounds as you tweak settings), you can choose Input Mon and Reamp to record the dry sound and have the Express process it.
You'll also likely want footswitches to control the looper (assuming autostart/stop or onscreen buttons are not cutting it or you). I used the $20 MVave Chocolate (in MVC mode) via Bluetooth to the iPad [The Pod Express's switches don't send Midi]
If your iPad has the old lightning interface, you'll need the "Lightning to Camera Adaptor"; a straight lightning to USB cable won't work.