using wp_cache in plugin

If you move to using BBpress 2, BBpress 2 uses custom post types and is a plugin of WordPress. This would let you do the above query using standard WordPress calls, and WP_Query. You would then automatically inherit all of the caching systems WordPress uses, and support for things like W3 Total Cache DB caching etc

Else, you could use the WordPress Transients API to cache the results:

http://codex.wordpress.org/Transients_API