No, I've never installed installous.
It's any app that does the in-app upgrade/purchace completely within the app.
I've tried with netstat, a tv guide app and fairlight.
(I only wanted fairlight but tried the other two to see if it worked at all.)
The fairlight developer was actually very interested, i sent him screenshots but he hadn't seen anyone else with the issue.
I was able to pay for pandora but that takes me to a web page where the purchase is done.
Netstat shows I have sandbox.itunes.apple.com... something like that (don't remmber exactly). open/listening. Must have been some app or jailbreak app that caused this.
ANyway, this is known to apple but seems to be limited to the developer environment. I am not a developer and have no beta version of any firmware.
From:
http://developer.apple.com/library/ios/#technotes/tn2259/_index.html
11 How do I resolve the "This is not a test user account. Please create a new account in the Sandbox environment." error message?
You are getting the "This is not a test user account. Please create a new account in the Sandbox environment." error message because you signed in with your iTunes user account when prompted by Store Kit to confirm a purchase. To resolve this error, sign out of the Store in the Settings application on your device and use your In App Purchase test user account when prompted by Store Kit to confirm a purchase.