You can't talk to BBM contacts because your iPhone won't have a BBM account. There are cross-platform clients available like PingChat, if you can get your BBM contacts to install it.
WhatsApp works quite well, looks and feels a lot like BBM. I got a bunch of my friends with iPhones and Androids to download it because I refuse to pay for extra SMS.
When I had my iPhone before upgrading to the BB BOLD, i used 'PING chat' a lot which helped me speak to people who had downloaded this app on the BB which meant you could speak to anyone who downloaded the free app and were given a name which people added you through(no one else could take that name) which i found quite handy because it made it easy to remember!
Matt
I use LiveProfile to communicate with people on different platforms. However, I really don't think you need an app. Not sure if this is what you're looking for, but I believe you just have to ask your BBM user friends add you as an SMS contact, and send your texts as you normally would. It will show to them in their BBM messages list. http://crackberry.com/text-messaging...erry-messenger
HTH