There is a lot of knowledge to be gained by reading the earlier postings on this thread.
For instance :
As well as their call sign, every ship has an IMO number
Queen-Elizabeth2 - IMO 6725418
http://www.vesseltracker.com/en/Ships/Queen-Elizabeth-2-6725418.html
As confirmed in a much later posting :
That's pleasantly surprising 
. QE2 appears to still have hers: IMO 6725418
Also some valuable general information :
Every powered ship over 100 gt has an IMO number. This hull number never changes, despite changes in ownership. Just been looking through my photo database but can not locate the folder with the numbers I have recorded for the ships I have sailed on. They are welded into the ships steel structure at various locations below deck (ie, on bulkheads, machinery spaces), prominent positions on deck, etc. Plenty of info on Google.
And the all important link to much more :
'the International Maritime Organization – is the United Nations specialized agency with responsibility for the safety and security of shipping and the prevention of marine pollution by ships.'
http://www.imo.org/Pages/home.aspx
Paraphrasing Skilly then, the IMO number is the equivalent of the chassis number of a car; it never changes, even though the registration (or in the case of a ship the call sign) may change.
