This morning (much to my surprise), the count of emails in my Inbox folder on the iPhone were up! So I guess that means that Yahoo! Mail Plus users DO get 'push' email -- yay. However, the Inbox was the only folder that at this point *seems* to be pushed to -- none of my other folders show any activity whatsoever even though they received emails.
Also, the lag time for when the emails are 'pushed' is huge. I *thought* the idea was that as soon as an email is received to Yahoo! it's 'pushed' to the iPhone, not stuck in a cue to sit for hours! Anyhow...
I did figure out something of a workaround using the new 'Add to Home Screen' option by adding the following icons/links to the Home Screen:
Y! Mail http://us.m.yahoo.com/p/mail
Y! Address Book http://us.m.yahoo.com/p/addressbook
Yahoo! Mail (Safari/web version) http://us.f329.mail.yahoo.com/ym/login?.rand=cgciblrrvn5cl
What the above allows me to do is get an overview of my Yahoo! Mail using the Safari version, then drilldown to any folder that has email using the mobile version for easier reading and tabbing through the mails. As for the address book, even though I successfully imported my Yahoo! Address Book contacts into the iPhone via iTunes, I prefer the Yahoo! mobile layout better. Though not quite what I had in mind convenience-wise with the iPhone and push Yahoo! Mail, this workaround at least gets me closer to it and makes the device a lot more useful to me.