Quoting Angela Byron <drupal-devel@webchick.net>:
Sure, I could go either way on this. NOTE, TODO, etc. are handy clues that you should pay attention. Core just was using "Note:" in some places and "NOTE:" in others, so Nedjo was trying to find a consistent way to show this. I think as long as caps aren't used excessively, this is fine for a few keywords.
To me when I read "Note:" it tells me that it is something the developer added to the comment section to remember but isn't necessarily important to further development; but, when I read "NOTE:" it tells me that the developer wanted to remember something that is very important to further development and that I shouldn't ignore it. I don't have any such cognitive idea about "TODO" vs "Todo" but Todo isn't an officially defined word and should be capitalized. I do agree that too much capitalization is the wrong thing but should be used to the benefit of others. Earnie