I have 15 non-consummable products.
Suddenly, all 15 of them started to appear as 'invalid' in one of my devices (iPod Touch 4G), but valid in the other (iPhone 4S); very weird indeed.
I deleted/reinstalled and now it works OK.
Someone said you must launch from Xcode, but this is not necessarily true. I have many beta testers with AdHoc builds and Test User accounts, and most of the time they seem to be able to purchase OK.
That said, the whole Sandbox Environment is always shaky and has plenty of "down time". Sometimes waiting is the answer.
