First of all sorry for misunderstanding with Alex Chiang, I was trying to 
collect some data from other i7 mobile users to try to isolate the problem 
before posting.

I tried different things: I disabled speedstep from the bios and the results 
where more sensible (higher priority lower execution time). The same thing 
happens if I disable throttling  by software selecting "aggressive powersave" 
settings.

So I suppose that is something related directly or indirectly to frequency 
scaling or turbo boost (unfortunately I have no bios option to disable only 
turbo boost) 
Here are the results of the same test of Peter Zijlstra

time  sudo nice -n 19 lame -b 256 -V0 -h youcantdothat.wav  2&> /dev/null

real    0m1.105s
user    0m1.090s
sys     0m0.010s

time  sudo nice -n 0 lame -b 256 -V0 -h youcantdothat.wav  2&> /dev/null

real    0m1.108s
user    0m1.100s
sys     0m0.010s

time  sudo nice -n -20 lame -b 256 -V0 -h youcantdothat.wav  2&> /dev/null

real    0m1.354s
user    0m1.330s
sys     0m0.000s

They are almost the same results that I obtained before.
Looking at top there are no other process that is using the cpu  (main are 
Xorg and kopete).

I tested this on  2.6.31-17-server (ubuntu 9.10) 2.6.31-17-generic (ubuntu 
9.10) and the default kernel of fedora 12 (live cd).
As soon as possible I'll try 33-rc5 as requested by Mike Galbraith (now I 
can't shut down the computer).

Ok, a last minute update! 
I slightly modified i7z to save a log to disk. If i7z output are reliable I 
suppose that it can show is the problem:
with nice set to 19 a processor 3 reach the maximum speed,  with nice set to 
-20 its maximum value is 300-400 mhz under the maximum value. (I attach the 
two logs named as the nice level used obtained running lame on a bigger file)
Please note that logs may have some value read before and/or after the 
start/end of the running process (lame).

The next update will be the results with 33-rc5 kernel.

Luca