Since RIM has officially announced BlackBerry 6, Part of the Press release states what devices are expected to be compatible with the new OS.
Compatibility with Existing BlackBerry Smartphones
BlackBerry 6 will debut on the new BlackBerry® Torch™ smartphone (announced today) available from AT&T on August 12 and it is also designed to run on select BlackBerry smartphones already in market. Subject to carrier certifications in the months ahead, the new OS is expected to be available for the BlackBerry® Bold™ 9700, BlackBerry® Bold™ 9650 and BlackBerry® Pearl™ 3G, as well as future BlackBerry smartphones
Source (Blackberry.com Press Release)