So, I tried streaming an audio file from my Pogoplug to my iPhone 4 and note that the audio stops when I switch to another app.  This tells me that the Pogoplug app for iPhone is not updated to take advantage of iOS4's multi-tasking.  So when can this update be expected?  It seems that the many other streaming apps that I have on my iPhone have been updated for some time now so it would seem a somewhat trivial update.