We were throttled on the UBC CMS with kick back traffic from Connect going down. Always impressed with how well our pretty basic setup can handle it. Properly tuned PHP opcode cache with a content cache still goes along way 🙂
WP Engineer has great post on WordPress and memory usage. Many users load up every plugin under the sun and do not understand that all though these plugins seem to be lightweight they can easily consume all of your PHP memory if your PHP max_memory settings are too low (which they often are with webhosts).
The two plugins mentioned in the WP-Memory-Usage by Alex Rabe and the even more detailed WP-System-Health by Heiko Rabe. Both work checking out to see if your blogs are getting close to maxing out on the php memory limits.