Jacob Santos‘ FuncDoc notes:
The WordPress Object Cache changed in WordPress 2.5 and removed a lot of file support from the code. This means that the Object Cache in WordPress 2.5 is completely dependent on memory and will not be saved to disk for retrieval later. The constant WP_CACHE also changed its meaning.
I’ve just started using the object cache and I’m happy with how it works now, so these changes are somewhat concerning. On the other hand, I’m confident that I’ll see even more performance gains once I get my head around the new cache and figure out how to implement it.
Extra: Peter Westwood’s Cache Inspect looks useful.