AMD Athlon 64 X2 3800+ running on Ubuntu 10.04 amd64. I'm running the CPU miner only on one core, because I end up with 100% CPU usage on the other core if I run pocblm 
Out of curiosity I just booted Windows Vista Home 64 on the same maschine and installed pocblm. The CPU usage is between 0 and 3% Furthermore Windows outperforms Linux by the factor 10 (3.5 vs. 33 Mhash/s)
I don't like that
My first guess was that you are CPU-mining, but that wouldn't make sense with 0-3% usage...
Have you installed the latest GPU & OpenCL-drivers? How high load has the card? Is there a significant increase in temperature?