Yahoo push works perfectly for me. I'm curious - for those of you who don't have it working, how did you initially set up your account? Sync from your computer, or did you go through the iPhone interface and pick Yahoo out of the list of providers? I did the latter, and it works great. I'm guessing it has pre-configured settings that makes it work, and if you sync from your computer it just defaults to standard POP or something.
iphone.
just clicked the yahoo logo. added my info.