When I'm indenting blocks of code like functions, conditionals, etc... I use tabs.
When I'm trying to get various parts of the code to line up (like when I have very similar lines of code and I want to emphasize their similiarity and clarify the different parts of each line, I use spaces (that way it doesn't get messed up if someone uses 4-space-tabs instead of my 8-space-tabs).
edit: Linus Torvalds endorses my 8-space-tabs? YES!
