cpuinfo shows wrong MHz value

[Joerg Platte <jplatte@naasa.net>]

I'm running kernel 2.6.28.4 and /proc/cpuinfo and powertop and vmware are 
unable to detect the correct speed (1.6 GHz) of the processor:

cat /proc/cpuinfo
processor       : 0
vendor_id       : GenuineIntel
cpu family      : 15
model           : 1
model name      : Intel(R) Pentium(R) 4 CPU 1.60GHz
stepping        : 2
cpu MHz         : 13000.000
cache size      : 256 KB
fdiv_bug        : no
hlt_bug         : no
f00f_bug        : no
coma_bug        : no
fpu             : yes
fpu_exception   : yes
cpuid level     : 2
wp              : yes
flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca 
cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pebs bts
bogomips        : 3197.21
clflush size    : 64
power management:

According to dmesg the kernel detects the CPU correctly during bootup:

Enabling fast FPU save and restore... done.
Enabling unmasked SIMD FPU exception support... done.
Initializing CPU#0
PID hash table entries: 4096 (order: 12, 16384 bytes)
TSC: PIT calibration matches PMTIMER. 1 loops
Detected 1598.609 MHz processor.

IIRC this behavior started some kernels ago, I can't remember which kernel it 
was. Is it possible that the ACPI information is wrong?

regards,
Jörg