Update from RIM: Unfortunately BBM 7 currently only supports OS 6 and up, so no love for lower OS devices just yet. Sorry!
If you happen to still be using an older device with OS 4.5 or OS 5, you can now update BBM to take advantage of the new features in version 7. While BBM 7 was rolled out a few weeks back to higher OS devices, there was no support for legacy devices included. Now it has been updated with full support for devices running OS 4.5 and higher. Unfortunately however BBM Voice yet available for these devices.