When you don't use busybox but build a rootfs with standard System V init (with the sysVinit package) /etc/inittab and all init scripts still remain for Busybox (no runlevel supported, etc). When busybox is not compiled and SysVinit chosen a compatible set of scripts should be provided in /etc.
This has been fixed by http://git.buildroot.net/buildroot/commit/?id=7e2bf63080cc0011c84ee46ee2aba161569154c5 This commit was not part of Buildroot 2010.11, but will be part of Buildroot 2011.02, scheduled to be released end of next month. The sysvinit inittab proposed is probably not perfect (for example it doesn't work well for halt/reboot), but at least it allows the system to boot properly. Feel free to suggest further improvements.