I do believe that's actually a good guess done by 'mcnobody'. If this is true, 'fabricating' a 'phony' IMEI is somewhat 'daring', to say the least (though possible). because to make it unique would require 'blind guessing', I think. The IMEI also represents the actual phone model, fyi (as this 'crucial' piece of information was missing).
I haven't used my N900 for a couple of months, now my daily driver is N9 and I am still able to use Whatsup, although I receive some messages with a slight delay. Do you guys have any idea why WhatsApp doesn't ban Whatsup users? If the reason was "wrong" IMEI, I presume that N9 users should also be banned, and yet they are not.