I don't know exactly but if a blackberry has Wi-Fi, and could connect to a router that has a printer on it network, I could see the possibility of it happening. But again, I am not 100% sure. Would be cool though. And if you can't, don't ever rule it out in the near future. They already have the Documents to Go App.
And with third party software, you have even more options. I can print attachments to a network printer at the office. It's useful if someone forwards you something during a meeting and you need to make printouts. I usually don't use it when out of the office since I don't like leaving things on a printer for someone else to pick up.