I am having same problem with HP G62 with 12.04, Jupiter 0.1.7 etc
took it apart and cleaned tons of fluff out of the fan , still same problem (but surely better for the computer anyway), then saw this thread!

I've found that powering the laptop down, unplugging, removing the battery , then putting battery back in and restarting gives a few blissful minutes of silence from the fan... until sensors reads 50 degrees, the fan kicks in and then doesn't turn off, despite sensors reading 27-30 degrees.... bios settings are set to FAN ALWAYS ON <DISABLED>
irritating bug this