To copy myself from the other thread:
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I wasn't familiar with BBM at all before this thread popped up, but now after some googling around, yeah, I gotta agree. Blackberries apparently all have a unique PIN assigned to them, written in the ROM, and impossible to change, and that PIN number is used to communicate with RIM servers. There is a BB Connect SDK which apparently allows one to generate a PIN, but working out how to use the SDK, compiling it for N900, and then figuring out the actual BBM implementation is a lot of work since atleast I simply cannot find a single application with source codes that'd allow one to connect to BBM.

So, first you'd need to find a way to get a suitable PIN number that the servers even accept at all. Probably doable via BB Connect.

Then you'd need to analyze all the packet data going back and forth between the servers and BBM itself, figure out the exact API, what kind of encryption you might need and all that. Months, if not years, worth of research.

Third, you'd need to create the actual application. Still worth atleast weeks of worth, most likely months.
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It's not the PIN that is the biggest issue. But yes, a PIN can only be active on one device at a time, and most likely if the same PIN was used by 2 or more BBM instances at the same time RIM would ban the PIN.