So it looks like it's not exclusive to Yahoo mail. I'd be pretty peeved if I got an iPhone and couldn't check my regular email accounts. I'm pretty sure Apple wouldn't do that to us. So maybe we don't have to worry about the quality of Yahoo's email going down - because most iPhone users will just use their regular email accounts anyway.
Yahoo is providing the push email services. I'd imagine that the iPhone could check other accounts, but you'd have to take the initiative to pull new emails from the server. If you go with Yahoo then, well, the emails are pushed out to you.