As far as i have understood, lurking these forums, it is a feature in the ATI driver that allows the GPU to use CPU processing poser when the GPU is at a high load (to allow for >100% GPU usage, so to speak). This feature, when combined with bitcoin mining, will cause each GPU to use 100% of one CPU core (if you have 3 GPU's and an 8 core CPU you will be using 3 of the CPU cores, or 37.5%).
So far there is no way to reduce this load on the CPU, except for setting the affinity of the phoenix processes (or m0mchil, for that matter). If you set all the processes to the same CPU core, only this core will be at 100% load, with no significant reduction of hashing power (taking the example from before you would go from 37.5 to 12.5% CPU load).
The way you do this is to locate each individual process in the task manager. If you right-click a process you can choose 'Set Affinity'. By default this will be set to '<All Processors>'. I guess the rest is pretty selfexplanatory - just set all of the processes to run on the same core..
This assumes that i just run the miner and leave it, trusting that it will not crash. Every time the miner is restarted i would have to set the affinity, which is not practical. Is there a way to launch an app from the command line, and tell windows which core to "prefer" for that process?