WiFi Edge / 3G Issue
This is a WiFi / Edge/ 3G Switching issue.
I've pretty much confirmed it, my brother works within iPhone support, and i've emailed him to look into it, but he's not available today -- i'll get him to comment on the issue.
When your phone refreshes with the Yahoo/MobileMe/Whatever server, the server realizes where your phone is (on the network) and how to reach it.
The issue is, let's say you are on WiFi (arn't we all, who needs to spend data plan MB's especially if you are like me -- ON ROGERS IN CANADA!!) and working with your iPhone, as soon as you hit SLEEP/STANDBY, the phone switches the WiFi off, and switches to EDGE/3G. Problem is, last time it spoke to the Yahoo/MobileMe server, it was on WiFi, so now, when the MobileMe/Yahoo box tries to PUSH you an email -- it can't, because your phone is "gone"
FYI -- this is pure speculation, but i've read many threads that talk about this exact problem --- but here's how to re-create it.
Turn off WiFi, and use EDGE / 3G ONLY -- and see if your push works now -- mine does. In standby I get email no problem, before I had to bring the phone out of standby before email would arrive.
Ideally to fix the problem, each time the phone switches data communication paths, from Wifi to Edge, or Wifi to 3G, or even Edge to 3G or the other way around, the phone should re-contact the required push servers to ensure the push connection is still "hot"
I've been using a blackberry since the original BB950, and push email has always been 200% reliable for me -- Sorry

but the iphone's email is simply NOT ready for primetime the way it's working, no business person is going to use a device for email that is flakey (even 1% flakey). The problem lies in the technology, Blackberry's push system is integrated directly with the Cellular Telephone Network, at the network level. The iPhone is not, therefore it's going to take alot more back and forth connection checking to make this stuff work.