As a matter of policy, Apple requires developers to test in-app purchases (IAP) with a test user account before the application in question can be approved. Unfortunately, this test account has been offline for a week now with no word as to why.The reason for the account error is unknown, as Apple has been silent on the issue.
Speculation has naturally touched on the Lodsys patent lawsuit threats over in app purchase and upgrade link functionalities, although reports that the IAP test account has been offline for a week now would indicate that the change predates the Lodsys notice letters that were first received last Friday. Consequently, Apple would have had to have advance notice of the letters if the two events are in fact related.
Update: TUAW now reports that the issue appears to have been resolved and that submissions for IAP-enabled apps are now able to pass through testing process.