Another data point... I had the problem using 2.6.15-26-386 with an Athlon CPU (see my comment above), then it went away once I moved to 2.6.15-26-k7, but now appears to have returned once I upgraded to 2.6.17.7-k7. All under Ubuntu 6.06. -- Power down after shutdown does not work... https://launchpad.net/bugs/43961