Currently, there are no risks to the propietary operating system that BlackBerry handhelds run on. However, that is not saying that there won't be in the future. You see the mess that the SymbianOS is undergoing now. RIM has done a great job in locking down the OS structure and environment, and since it does and most likely will remain propietary, I would think the chances of a risk in the near-term are extremely slim to non-existent.
But like you, I have not seen an 'official' statement from RIM concerning the risks. You're security department should be more concerned about the security of the handheld, considering the data that passes through it and what it does and doesn't have access to, rather than the possibility of it being infected with a virus. When they see a POC (Proof-of-Concept) virus for the BlackBerry, then I would worry. The only time you will see anything relating to BlackBerry and virus within the next few years will be with other devices running on other OS's running BlackBerry Connect software.
In the beginning the Universe was created. This has made a lot of people very angry and is widely regarded as a bad move.