I could not get Opera Mini (4.2 or 5) to work on my BlackBerry 8100.
Open the Blackberry browser instead. It should drop you on an ugly-as-sin WAP homepage and allow you to navigate from there. That's the best you get.
HOWEVER, Rogers just announced an upgrade to their Pay As You Go browsing that gives access to more apps. With any luck, SpeakOut will make a similar change (the switch is scheduled for March 11th on Rogers). That might allow the use of Opera Mini.