You said it Sam.
To forgo Yahoo's login page calls for a black hat handy :P.
Merawwa, I would advise you to stay clean and straightforward, go thru the channel, use Yahoo's authentication API. Use them to let the user login normally, (now here you can have a bit of a say over how much of the login page you wanna show, modify the browser window parameters), then you can redirect to mail.yahoo.com to get to the inbox or where ever it is you wanna redirect your user to.
Learn about these APIs here