I have a CentOS 5.5 installation with the stock CentOS 5.5 kernel. I have modified the init script in the initrd, commenting out some unneeded modules, lowering the interval time of the "stabilized" ...
On my CentOS 5 workstation I get a few seconds wait after dm-raid45 has been loaded ("Initializing Driver" or something like that). This seems to be part of the initrd. After that the system boots ...