UPDATE: Looking at Cache Backport, this module seems somewhat ridiculous ... I have now abandoned it.
Provides multiple caching backends framework. Use any drupal 6 cache.inc file for a given cache table.
This module works by generating php source-code in order to separate cache_get(), and so on, into different "namespaces" by prefixing the function names.
As this module auto-generates code, it might not be considered production-grade. Use with caution.
The following example uses the memcache backend for all cache tables except cache_form: