The list name in the Address Book is available in Mail as a distribution list. Begin typing the distribution name, and you can choose it from the list of addresses. In addition, if you select the distribution list in the To: field of an email, you can expand the list to explicitly list the email addresses in the To: field.
[robg adds: This was submitted as a 10.5 tip, but it exists in 10.4, too (and perhaps earlier). I'd never noticed the menu item, though, and judging by the feedback from the queue reviewers, many others may have missed it, too.]

