This bug also affects me.
When on battery, cpu frequency scales properly on-demand 1.6 to 2.8 GHz.  (current policy shows range of 1.6 to 2.8)
When AC adapter is plugged in, cpu frequency is locked at 1.6 Ghz.  (current policy shows range of 1.6 to 1.6)

-Computer-
Processor		: 2x Intel(R) Core(TM)2 Duo CPU     T9600  @ 2.80GHz
Memory		: 4111MB (543MB used)
Operating System		: Ubuntu 11.04

cpufrequtils 007: cpufreq-info (C) Dominik Brodowski 2004-2009
Report errors and bugs to <email address hidden>, please.
analyzing CPU 0:
  driver: acpi-cpufreq
  CPUs which run at the same hardware frequency: 0 1
  CPUs which need to have their frequency coordinated by software: 0
  maximum transition latency: 10.0 us.
  hardware limits: 1.60 GHz - 2.80 GHz
  available frequency steps: 2.80 GHz, 2.80 GHz, 2.13 GHz, 1.60 GHz
  available cpufreq governors: conservative, ondemand, userspace, powersave, performance
  current policy: frequency should be within 1.60 GHz and 1.60 GHz.
                  The governor "ondemand" may decide which speed to use
                  within this range.
  current CPU frequency is 1.60 GHz.
  cpufreq stats: 2.80 GHz:5.64%, 2.80 GHz:0.28%, 2.13 GHz:0.35%, 1.60 GHz:93.73%  (4292)
analyzing CPU 1:
  driver: acpi-cpufreq
  CPUs which run at the same hardware frequency: 0 1
  CPUs which need to have their frequency coordinated by software: 1
  maximum transition latency: 10.0 us.
  hardware limits: 1.60 GHz - 2.80 GHz
  available frequency steps: 2.80 GHz, 2.80 GHz, 2.13 GHz, 1.60 GHz
  available cpufreq governors: conservative, ondemand, userspace, powersave, performance
  current policy: frequency should be within 1.60 GHz and 1.60 GHz.
                  The governor "ondemand" may decide which speed to use
                  within this range.
  current CPU frequency is 1.60 GHz.
  cpufreq stats: 2.80 GHz:6.37%, 2.80 GHz:0.09%, 2.13 GHz:0.11%, 1.60 GHz:93.44%  (2654)
----
cpufrequtils 007: cpufreq-info (C) Dominik Brodowski 2004-2009
Report errors and bugs to <email address hidden>, please.
analyzing CPU 0:
  driver: acpi-cpufreq
  CPUs which run at the same hardware frequency: 0 1
  CPUs which need to have their frequency coordinated by software: 0
  maximum transition latency: 10.0 us.
  hardware limits: 1.60 GHz - 2.80 GHz
  available frequency steps: 2.80 GHz, 2.80 GHz, 2.13 GHz, 1.60 GHz
  available cpufreq governors: conservative, ondemand, userspace, powersave, performance
  current policy: frequency should be within 1.60 GHz and 2.80 GHz.
                  The governor "ondemand" may decide which speed to use
                  within this range.
  current CPU frequency is 1.60 GHz.
  cpufreq stats: 2.80 GHz:5.58%, 2.80 GHz:0.29%, 2.13 GHz:0.35%, 1.60 GHz:93.78%  (4371)
analyzing CPU 1:
  driver: acpi-cpufreq
  CPUs which run at the same hardware frequency: 0 1
  CPUs which need to have their frequency coordinated by software: 1
  maximum transition latency: 10.0 us.
  hardware limits: 1.60 GHz - 2.80 GHz
  available frequency steps: 2.80 GHz, 2.80 GHz, 2.13 GHz, 1.60 GHz
  available cpufreq governors: conservative, ondemand, userspace, powersave, performance
  current policy: frequency should be within 1.60 GHz and 2.80 GHz.
                  The governor "ondemand" may decide which speed to use
                  within this range.
  current CPU frequency is 1.60 GHz.
  cpufreq stats: 2.80 GHz:6.27%, 2.80 GHz:0.09%, 2.13 GHz:0.11%, 1.60 GHz:93.53%  (2720)