To see what frequencies are available:
My Centrino 1.8GHz also goes as low as 600MHz, no lower. May be a limitation in the cpufreq code. When I tried FreeBSD, after a lot of fuss I got it down to 200MHz, which was nearly unusable.
I'm not sure that you can go lower, but you can edit the values in the files in /sys/devices/system/cpu/cpu0/cpufreq/ (be careful)