perhaps it's not as easy I thought..

to me it shows the current frequency, although I've got a smp kernel. (2.6.18)
Google said the same for most users.
e.g. http://docs.huihoo.com/gentoo/resources/document-listing/power-management-guide.html
So I'd guess it's dependent on the kernel version, I wasn't able to find something more specific in the kernel documentation.
A short research in the kernel sources didn't answer this, too.

What google also told me..

There is more info in the files
/sys/devices/system/cpu/cpu0/cpufreq/cpuinfo_cur_freq
/sys/devices/system/cpu/cpu0/cpufreq/cpuinfo_max_freq
/sys/devices/system/cpu/cpu0/cpufreq/cpuinfo_min_freq

cpuinfo_cur_freq is only readable by root on my system, so this could lead again into problems.

At least my question how the max frequency can be determined has been answered. :-)