Ordered lists: numbers or letters?

I'm working on a set of guidelines for the use of user interface elements. I'm currently working on ordered lists (<ol> elements) and I have a couple of questions which I've been unable to find a definitive answer to:

• Under what conditions is it more appropriate to use numbers (decimal, lowercase roman, uppercase roman) VS letters (lowercase, uppercase) to number list items?
• What's the difference (if any) in terms of usability between the different types of numbers and letters available?

I've searched for previous research / data on this, but I've been unable to find relevant information. Any help or friendly points in the right direction would be greatly appreciated!