I recently booked a Business Objects Data Integrator exam (DMDI201) at the Pearson Vue website.
On this occasion I noticed that the Pearson Vue website now requests that you enter your SAP User ID:

Pearson Vue SAP User ID Screen
Submitting my SAP User ID allowed me to continue with booking my exam.
If you don’t have a SAP User ID, how can you book an exam? In the first instance, I suggest you contact your regional SAP Support office. They should be able to set up a SAP User ID for you.
This post on the Business Objects forum suggests that people are able to get their SAP User ID’s by contacting support.
Give them a try.
August 7, 2009 at 9:45 am
You can sign up for your own SAP ID at sdn.sap.com. I had not noticed the change to the Pearson Vue site, but this number allows you access to a world of BI & BOBJ information, including ALL the BOBJ manuals at http://help.sap.com/businessObject/product_guides/ and a huge pile of demos, white papers, trialware, etc.
I have been working with SAP BW/BI for ten years and I have almost never gotten any results by contacting a regional office.