This is actually a special case of a general concept: specifying how much you trust particular addresses. Instawallet has just coined 'green address' as a nickname for an address that you will accept payments from instantly (0 confirmation), but what do you call an address for which you would prefer to see 1 or 2 confirmations (~10 or 20 minutes, respectively)?

If Bitcoin UIs allowed you to specify how much you trusted particular sending addresses (in terms of confirmation numbers), things like green addresses would fall out naturally. Also, providing these trust numbers is a great usecase for a web-of-trust system.