I'm wondering about the practical outer limits of what drupal can handle. I have a client who currently has a site with about 40k regular users, and wants to blog-enable them. I'm reasonably confident that I can handle the user detail sync stuff, and have a few ideas about mysql changes that might be needed, but I'm wondering what I've not thought about.

Is anyone out there running a site of that sort of scale with drupal? If so, got any words of wisdom to share?