According to this comment, the debug.log file in the Bitcoin application data directory should be truncated on startup if it is bigger than 10MB. I'm running Bitcoin 0.5.2 on Windows 7 and it does no such thing - the file keeps increasing in size even after restarts, and it is already over 1GB.
Why is it not truncated? Is it safe to stop Bitcoin and truncate the file manually, or delete it completely?