you would not even have to release the source code for them to figure out if you are using an n9 - you are.giving them your imei, which they can run in any public database (or use an automatic script) and just bar all imeis that come up as Nokia n9's if they really wanted... releasing the source would make development quicker but that is up to the original author at the end of the day!
The java port of whatsapp let you write your IMEI, and could be fake. This could be a solution to avoid problems. I'm not sure, but I think that the IMEI is just used to generate by md5 the password for logging.