I was trying to run more than 8 Centos 5.3 virtual machines using VirtualBox 3 on Ubuntu 9.04 and I was running into high cpu load (80-90% cpu load when the virtual machines were idle).
After searching a bit around the web I've found a bugzilla thread where some people smarter than me figured out that the high cpu load was caused by the default CentOS 5 kernel settings.
Without going to technical (you can read the details in the bugzilla thread) here are the steps that you can use to fix the problem.
Add the CentOS testing repository at the end of the /etc/yum.repos.d/CentOS-Base.repo
nano -w /etc/yum.repos.d/CentOS-Base.repo
paste the following repo and save
Install the "fixed" kernel
yum install kernel-vm
Edit grub.conf to boot into the new kernel automatically.
nano -w /etc/grub.conf
The newly installed kernel should be the first on the list.
You need to edit the grub.conf entry to say 0 instead of 1 (the first entry is 0, the second one is 1 etc.)
Reboot into the new kernel
shutdown -r now