Establishing a permanent global employee id

Question

Greetings,
Let me preface this by saying that I am new to SAP, new to HCM, and new to the forums.  Apologies if I do not have all the terminology down quite yet.
We are getting ready to kick off our HCM implementation with deployment of OA and PA.  We will be relying on consultants for the technical work.
One benefit that we hope to derive is that we can create an employee ID number for each employee that stays with them for their career.  This would essentially become a primary identification key that we share with other systems and partners.  Thus, no matter where you go, you identify yourself by your WWID (World Wide ID #).  This number should never change, just as your SSN never changes.
I have learned that the way SAP HCM works is that each time an employee moves to a different entity (company code, overseas assignment, etc.) his personnell number changes.  That seems odd to me but it is what it is. 
Given that the personell id could change, what would be the best approach to creating and maintaing the permanent id number?
Many thanks,
Mike

Answer 1

Thanks for the replies.  Msyelf and our HR department are still grappling with why an employees personel number should ever need to change?  I guess it is what it is, be it a good reason or a design flaw.
It seems like one problem with the Central Person ID is that this is, for the most part, a hidden field almost everywhere in SAP?  Can I easily look up an employee base on Central Person?  Can I run reports on the Central Person?  Given that an expatriate employee who returns home will now have 3 personell numbers, can I even run a report that shows his/her employment history over time?
This is one of those seemingly little things discovered early in implementation that have people going, "Dang, did we make a bad choice here".  Hopefully, we work through it.
Thanks again.
Mike