I see that these posts are old now, but I have some info on this.
Basically, the HTC Mail that comes with the phone (EVO, in my case) can handle yahoo just fine. In fact, I DID NOT have to enter any extra "set up" info beyond my id and password. I thought I was going to have to know the various mapping info but NOT with the yahoo account.
You can set it to handle info in many different ways including a UNIQUE Notification SOUND for that e-mail as opposed to the gmail app notification sound for that account.
The mails do download to your memory, so be careful if you run low when it comes to how many days are on the phone.
I find the HTC Mail app FASTER than the Yahoo Mail app. I actually found the yahoo mobile web page faster than the app and had been looking at mail that way.
What I don't find so far is easy manipulation of folders with the HTC Mail and yahoo account.
Using HTC Mail Widgets also can be set for different accounts including any exchange accounts and thus show different "unread" numbers on them along with the name of the account.
I hope you find this helpful.