A long time ago I announced that I had built up a PHP framework for my job that was designed for high-traffic websites. It was modeled after Ruby on Rails, but let you scale a bit more easily using things like memcached and MySQL master-slave configurations (Rails probably supports these features nowadays, I just haven't paid much attention to Rails in about a year after I stopped working with it).
I finally decided to get off my ass and remake this framework (I couldn't use the old version because it was not my intellectual property, however there isn't anything stopping me from just using the same ideas). Unfortunately the current incarnation doesn't really have any of those nice scaling features, although I don't think they'd be terribly difficult to add. Adding master-slave configurations would just require a bit of tweaking on the database connection class, and the cached-model-identity-map system that it had would require making a subclass of Model that uses memcached in addition to the database.
You can grab the current version from Github here. I will warn you though, use at your own risk! It isn't very well tested yet and will probably have security holes and random bugs. Also there isn't much by way of docs yet, so you'll have to figure out how it works on your own. If you've worked with Rails before then it shouldn't be too bad, it's just a matter of figuring out what you can't do anymore (like associations, migrations, lots of helpers, support for anything other than MySQL, ...).
What's the selling point? There isn't really one. This framework is pretty simple. It doesn't have many features, and it doesn't really enforce too much. It has a basic MVC structure, pretty URLs, a simple ORM layer, and...well not really anything else. The focus is on getting out of your way so that you can do what you need to do.
Anyway if you want to fiddle around with it feel free, and if you find bugs/problems or you have a feature request or something, please post it on the Github issues page.