If you haven't heard already, Google with be and is offering IMAP for their Gmail service! You might not have it available on your account immediately, but it should be within a few days or a week.
I'm one of the lucky ones that already has IMAP set up. I tried it out in Thunderbird and the setup was quite smooth. The only problem I ran into is that some of my old chats came up with a timestamp of 2:33am this morning. So, they were out of order and weird, but it wasn't that big of an issue.
The real point I wanted to make in this post is that this will be a killer (imo) for Yahoo mail. Back in the day when I signed up for Yahoo mail, they were on the leading edge for features and interface. I remember when they moved to the interface they have which they now call their 'classic' interface, and I have saved HTML pages that show what their REAL classic interface looks like.
But until now I have not had a reason to move to Gmail. While I do have a Gmail account, I don't use it as my primary account because there was no real advantage to switch to it. Some of the features might be nicer and sexier, but webmail is still webmail. Now that Gmail offers IMAP along with a host of other features that set it apart from other webmail clients, there is a huge reason to move to Gmail. For this reason, Yahoo will lose many 'techie' customers it hasn't already lost unless it offers the same feature.
In case you're curious, here are the settings: