The previous two replies don't answer what Ryan wanted to know. There is a difference between default SMTP account and default email account.
If you want to change what email address is preselected when composing a new message, you need to change the default email account:
"Tools->Account Settings...", select (highlight) the account you want to be the default, then use the "Account Actions" menu (bottom left-hand side) to select "Set As Default".
Note: As Ryan noted in this comment, the preselected address also depends on what folder is currently displayed. Though if you're not currently in a folder linked to a specific account (e.g. unified inbox), the default account is preselected.