Exchange 2007 is an awesome product. I wanted to be able to use the automatic OWA login when a computer is logged into the domain with valid credentials, but have the ability to use forms-based login when you wanted to access another account that wasn't the currently logged in user. So:
(for 2007 site only, this might need to be done different in a mixed 07/03/5 env):
1. Create a new website (called FormsAuth in this example) in the IIS Manager on your Exchange box. (eg webmail.company.ca) This will be used for the forms-based login.
2. Create DNS A record for your new domain (and website)
3. Run in Exchange Management Shell-> New-OWAVirtualDirectory -OwaVersion:Exchange2007 -WebSiteName "FormsAuth"
4. Go into Exchange Management Console. Set owa (Default Web Site) to 'Integrated Windows Authentication' under the "Authentication" tab in properties.
5. Set owa (FormsAuth) to forms based login.
Now when you go to webmail.company.ca you will get the form login. If you go to the
http://servername/owa it will log you in automatically.