I spent a good bit of time on the phone with Yahoo and trying just about every work around you could think of. Here's what I posted in another thread (I hope it helps)...
Push works on @yahoo accounts, but not yahoo accounts that are @yourbusinessneame.com. The yahoo accounts support IMAP, the Yahoo Small Business accounts only support POP, hence the pull, instead of push. (And from what I've read, IMAP isn't even a "real" push...whatever that means).
More simply put, this is not a Blackberry. If you want instant e-mail, you're not going to get it. Even the push for @yahoo.com e-mail addresses isn't as fast as on a blackberry. The folks I talked to at Yahoo said that RIM monitors Yahoo's mail servers independently to make sure you get the instant "push". They said that's not the case with iPhone and Yahoo - it's just IMAP.
If you have IMAP or an IMAP enabled exchange server it seems you can get a better than "every 15 minutes" e-mail solution. However, it probably won't be instantaneous.
As for forwarding to your @yahoo account, I've tried it a few times and haven't had luck with it.
There were times I got almost instant push from @yahoo.com accounts. However, I'm thinking it was luck. I tested it repeatedly and there was no consistency to the speed.
To save you some time, here are some failed work arounds I tried:
I thought maybe I should just run everything through my @yahoo account. I went and paid the $20 to sign up for mail plus so I could send from all my accounts from @yahoo (even though I pay for the two other accounts; though I should get this back).
In any case, from there I thought I'd set the @yahoo account up to check those two accounts. That didn't lead to a "push" effect because it was way too slow. It wasn't any better than just checking every 15 minutes.
For the second alternative I thought I'd forward my other Yahoo accounts to my @yahoo account. That too was a flop. It didn't work any faster.
There's also been some talk about using .Mac for your push technology.
I spent about 30 minutes or so checking that out. As a caveat, I was doing it with a trial account, not a full one. I've tested the push speed with an e-mail sent straight to the .Mac e-mail address and to another account checked by .Mac. In each case, no dice. The push isn't "Blackberry" speed.
I did have a few times where it'll be almost instant. However, this brings to mind the old line from college that "correlation isn't causation." I think sometimes it just happens quickly, it's not an actual push.
Long story short, for the casual e-mail user this is a great phone with a good e-mail solution. However, if you need instant e-mail like you're used to on a Blackberry (journalist, politico, overworked-lawyer, etc.,) you might not be pleased.
If I'm wrong, please correct me. (I'd love to be wrong and find a Blackberry-like push).