The following message may be received when you attempt to create a new BlackBerry Internet Service account from a computer browser using a PIN and IMEI Number:
Your account is not accessible via HTML browser. Please use your device to access BlackBerry Internet Service.
This message indicates that you have already created a BlackBerry Internet Service account using the new Automatic Login feature of BlackBerry Internet Service 2.4. This is by design.
This new KB covers it:
BlackBerry Search Results
Damn stupid if you ask me but what do I know?!