My investigations shows that problem may be in driver version.
When I was using Ubuntu 10.10 and NVidia 260.19.36, there are no problems - poclbm was using only 0-5 %
Today I upgraded to Ubuntu 11.04 and fresh NVidia driver 270.41.19, and now poclbm (and phoenix too) are using up to 100%

When I install old NVidia driver 260.19.36 on Ubuntu 11.04, poclbm is digging and "top" shows low cpu usage (0-5%), but unfortunately X server cannot starting complaining about "ABI version mismatch". Now I am on Ubuntu 11.04 with fresh driver, and also seeking for better solution... Maybe I'll have to go back to Ubuntu 10.10.
EDIT: videocard is - nVidia Corporation GT215 [GeForce GT 240] (rev a2) ~20.000 khash/s