When using the OAuth2.0 User Password flow and having Delegated Authentication Single Sign-on enabled, will Salesforce by default use the Delegated Authentication to validate the user/password?
Yes. Salesforce.com does not store check the password for users with Delegated Authentication. The mode of authentication does not matter (e.g. OAuth2 versus API versus Apps). Any attempt to log in must be done through the delegated authentication web service. The first two list items describe what you're looking for:
When a user tries to log in—either online or using the API—Salesforce validates the username and checks the user’s permissions and access settings.
If the user has the “Is Single Sign-On Enabled” user permission, then Salesforce does not validate the username and password. Instead, a Web services call is made to the user’s organization, asking it to validate the username and password.