I have never ever got this feature to work.
Now that I have a mobile data plan I thought I would try to get it going again. So I tried all the multiple suggestions on all sorts of websites suggesting reset this and reset that. But to no avail.
I even made a new Yahoo! Account but again no Push. Even worse having Push on sometimes means the Backup 15 min fetch doesn't even poll/fetch. So I have it back to 15 fetch with push off.
This is supposed to be supported on free Yahoo mail but I have had issues since 1.14 and 3.0 has not solved it.
I now have a jailbroken iPhone 3G running 3.0.
Any ideas?