There's a lot of ways to update twitter using your blackberry; install a twitter client, open twitter mobile in your browser, update via email. The 3rd one is an option people often forget or even think is possible.
Your Blackberry is made for email, why not make updates or even uploads via email. I'm a big fan of this concept and I continuously look for companies that offer these services.
One company that stood out is called Posterous. Their services enables you to update your twitter account by simply sending them an email to update twitter. You can just write your post on the subject line of your email and that's it. Posterous even shortens the links in your post. Using posterous you never have to install any clients or have to browse their mobile version that takes time to load. Currently Posterous support uploads to Youtube, Video, Flickr or post updates to blogs using blogger and wordpress.
Download Update Twitter Use Your Blackberry's Email via http://www.posterous.com/