You could replace the Login.aspx (and Register.aspx if needed) with your own custom version that checks first against LDAP and then against the db if auth fails in ldap.
You would keep it in a custom project and use a post build event to copy it up to replace the included version.
After validating the user you would just use FormsAuthentication.SetAuthCookie or FormsAuthenctication.RedirectFrom login.
I will ponder this scenario for the future, this is the first time anyone has asked for this.
Hope it helps,
Joe