I have been waiting for more comics apps to hit App World for the PlayBook and BlackBerry smartphones. The latest to be released is from Studio Aleph and lets you read comics on your BlackBerry PlayBook. The app is capable of opening CBZ, ZIP, and Image folder comics though not the CBR comic file format.
If that fits you bill you will want to check out Comics Reader in App World for a relatively inexpensive $0.99. Hopefully they will add CBR support in the near future!