I have a 45GB system drive in my production Vista workstation.  I've noticed for a while that I was running out of space (the space free bar turns red in Explorer), but I only install programs on C: and that seemed strange to me.  On a Friday afternoon I finally decided to investigate, and found hiberfil.sys sitting in my C: drive, even though I'm clearly running Vista on a high-end desktop workstation, and have my power settings configured to "High Performance".  When I verified that all Sleep options were set to "Never" within the Control Panel GUI, I went Googling and found this command:

powercfg -h off (run from a Command Prompt, using Run As Administrator)

The second this command was executed, hiberfil.sys disappeared.  Ahhh...  Peace, love, and no more red drive space bars...