I've accomplished quite a bit in the last week.
On top of my day job, I cleaned out my basement, fixed my car, and got Xen working on my server.

As for the project, I feel I have a solid understanding of how the current cache works and exactly what needs to go into the interface. I'm a bit worried about the invalidate_by_MyISAM_filename stuff because I'm not familiar with the storage engine code, but everything else appears to be smooth sailing.

I've solved the interface problem I was researching: by avoiding it completely.
An interface in comments alone is sufficient at the moment with only two cache backends, a simple #define can switch them at compile time without any performance hit. This can fairly easily be improved upon by future developers so long as I document appropriately.

I've got a skeleton Query_memcache class going with all the necessary methods defined and commented with what needs to be done. Since not everyone will be wanting to compile in memcache with MySQL I'm thinking of expanding of the existing Query_cache #define's to integrate the memcache stuff in. I've looked into how to write a plug-in but I'm hesitant to go that route yet, if I can refactor working code at the end of the project without breaking anything then I might submit it as a plug-in as well.

I still need to figure out exactly how MySQL starts up and initializes everything so I can add in a few things to init my code. The tools cat, grep, and less only get one so far...

4 weeks till midterm evaluations and I feel worlds better after this week.
I really feel like I'll make it now.

My short term goal is to have working code that compiles cleanly, connects to a memcached server, and can cache things without breaking anything.
Further goals are more robust code, quality tests, better performance, complete documentation.
Lofty goals would be to refactor it into a plug-in, caching across multiple servers, and make fries.

Estimates:
2 week - code connected in and tested, caches but never hits.
4 weeks - caches, hits, working list of bugs
6 weeks - iron out most bugs, clean things up
I'll try and rework these estimates as time goes on.

The weather here in New England is currently hot and muggy, not the best of weather for coding, but a cold beverage and a comfy chair in the basement makes up for it.