Had another look at it too,
It does not seam it would allow for multiple speaker output with airplay from iOS devices. And at almost tree times the price of an airport express, which has lots of other uses (USB print server, wireless hotspot, wifi->ethernet bridge and more), one would be able to waist a few bucks to rather buy this.
When that is said, id love to have one for every room of the playGo!!
I always thought you could do it with the stock iOS. I was going to buy an airport express to stream to along with my apple tv. I know this isn't the answer you're looking for but could work for now. Use the apple iTunes remote app on your iPad or iPhone and use iTunes on your computer. I know that way you can select multiple airplay devices from your phone but the music has to be on your computer. You can also change the volume to each output individually. Hopefully apple will update airplay in iOS 6!
Oh yeah if you're looking to stream music from pandora, spotify, or some other source you can use Airfoil on your computer. It also let's you turn old iPhones or iPod touches in airplay receivers. I bought it to use with an old iPhone 3g. It's a paid app for osx and free for ios. Hope this helps until there's better options.