For those reading this post/answer wanting to use a BlackBerry - specifically a BlackBerry Curve 8520 - with Straight Talk, here is my experience. Voice and text work great but Straight Talk data cannot be used on BlackBerry phones. I installed/activated a new SIM and activated a $45/month unlimited plan, but when you try to change the APN settings, that's where you have problems. The BlackBerry Curve - can't speak to other BlackBerrys - won't allow the APN changes that Straight Talk provides to get data to work.
If someone has had a different experience, or Straight Talk has a solution, please pass along. Otherwise, if you want data, don't try using a BlackBerry on Straight Talk.