I have this very old school habit left over from my days of shooting of film; I still have the mindset that it is a sin to shoot with an ISO higher than 100! Occasionally, if it was an extreme emergency, I might use a roll of 200, but absolutely never anything higher. And my preference was for chrome with an ISO of 50 or 64 (depending on whether it was Fuji or Kodak), or even 25 for landscapes. Wow, that stuff was incredible.
I need to move beyond this mental roadblock. Even with 2.8 lenses, I run into situations where I have limited light and get frustrated that I can't get the shutter speed I want, and I never, never think to bump my ISO. It's just not a part of my thought processes.
In 2008 I will make a conscious effort to remember to try bumping my ISO when I run into those situations (maybe to 200