Michael Wales has taken a look at what effect HipHop could have on your CodeIgniter application (or lack there of).
So, what does this mean for the CodeIgniter community? In short, absolutely nothing. Most CodeIgniter developers are building applications that will run on shared hosts, virtual private servers or a cloud-based virtualization system. Of that very large group of our community, an extremely small number have the capability to compile the HipHop binaries or alter their configuration in order to serve HipHop pages.
He points out that, for most developers and applications, time is better spent optimizing the actual application - things like reducing the I/O needs, caching, etc. HipHop, unless you have a very high demand and load on the application, won't give much of a gain. He gives the example of Facebook's load and how even it would only relatively recently would benefit from the tool.