Just got a BB 7100 from new ( in the UK) it asked for a PIN when I 1st started the unit, I entered 1234, and it told me it was incorrect, I then rang the supplier who send that the blackberry default pin is 4321, I entered this and it stil told me it was incorrect, Now I have one attempt left,
What happens after this 3rd attempt if the PIN is incorrect ?

It's probably the SIM PIN, can you try a different or new SIM card? The SIM will lock and you will need to contact the Service Provider for a PUK code to unlock the SIM. (not to be confused with unlocking the BlackBerry's Provider Subsidy Lock)

Did they supply the SIM or is the SIM yours? If it's the latter, you should know what the SIM PIN is...

I rang the service provider again and someone else gave me a new default pin to try and that worked