I've only just started thinking about connecting my iDevices to an external speaker for a more portable setup - like partying outside somewhere. This little unit looks like a decent choice;
It's the MiniRig MR115 portable loudspeaker. It's rechargable by USB (1 charge lasting 50 hours!) with 15W output and a quality 3" driver.
Full details can be found here. They are priced at £85 shipped from the UK.
I've just ordered one, so I'll let you know how it goes with an iPad & iPod Touch!
*UPDATE* - The MiniRig arrived this morning, excellent build quality and superb sound - amazing given it's small size. I can plug this into the iPad directly and have a mobile party immediately! I'm also going to be using it with my Tenori-On and OP-1...but only when everyone's out of the house!