Yahoo does (or at least did) allow imap access. But it's always been crappy. This caused all kinds of problems for Windows Phone 7 as yahoo's imap implementation is non-standard. I've reverted to their android app but I'm not a fan of it. It auto-downloads images (allowing email marketers to track which emails I've read and slows the app down as well as using up bandwidth), I've also had problems with it sync'ing consistently and I find it a bit cumbersome to use. Kinda the pits since I've been using yahoo mail since around 2000, but between the mobile issues and the fact that they don't have ssl for their web mail client... I'm about ready to abandon them. Somebody needs to kick those guys in the tail.