I thought I would post what I did to get my Yahoo account to work with Gmail. I know Gmail is the way to go but I still want to keep my Yahoo account. I went into my Yahoo account and set it up to forward my emails to my Gmail account. I have tried the installed Yahoo, market Yahoo and k9 without much luck as it would stop notifying me of new mail and some sync issues. All my Yahoo mail is now forwarded to my Gmail and doesn't archive at all on my Yahoo account. I found this article after I set mine up and thought it might help some of you that have Yahoo. Like I said, I set my Yahoo mail to forward to my Gmail address as this pushes the email right away. You can also set up a pop3 within gmail instead of setting up to forward to an email address within yahoo but this will only make it fetch every hour instead of push.
How to read Yahoo Mail in Gmail Picobit
How to read Yahoo Mail in Gmail Picobit