IT WORKS!!!

And not a moment too soon.
I've finished my day job for the summer and have been pouring over the code the last few days trying to isolate the last two bugs.
I've also received word that I don't have 8 days left, I have 1...so I'm a bit rushed but still hopeful.

As part of my bug fixing for the hit code (which works beautifully now) I was able to remove all of the unnecessary mutex locking from within the cache. This is a nice speed boost as there is no longer any need to lock and unlock when working with the cache. It just works (tm).

I have a fairly rough implementation of the invalidate code that looks something like this:

if(anything changed) {
nuke_the_cache();
}

It works, and it allows for multiple instances to use the same cache.
My next task is to make this a bit more refined (per table invalidation) by using 'namespacing' within memcache. The idea works like this:
For each table, store a "0" in memcache with 'db_name:table_name' as the key.
Each time a table changes, increment that key.
When formulating the key for a query result, include each relevant get('db_name:table_name') in the key hash.

The nice thing about this approach is no matter how much data is in the cache,
invalidating is always O(n) where n is the number of tables involved.
The previous query_cache could slow down a MySQL instance in a write-heavy environment due to searching for and invalidating entries in the cache when a table changed.
With this implementation, everything is non-blocking and when a table changes a quick call to 'memcached_increment()' is all that's needed. Memcached will remove stale entries after they've expired, since once the namespace has changed they will never be read again.

It looks like I will not have time to refactor this into a plug-in during the summer of code timeline. But I've met and exceeded most all of my other goals so I'm not too worried.
I'll have a tarball, a diff, and hopefully a checkin with bzr done tomorrow to prepare for my final review.

Best of luck to all!
We're almost done!