Definitely.
As a former blackberry user, email (if it isnt too long or too big in size) can be completely viewed while the wireless is off or while you arent in the service area (in a tunnel, basement, etc). On the iPhone on the other hand, to read the email, you need to be connected to the ATT network and wait for the email to load (or download). Is it possible... that when the email is pushed to the device for the email to be almost... downloaded automatically so that having enough signal strength has nothing to do with viewing the email?
I spend a lot of time on the subway in nyc, for example, where there is no service. I noticed today... that because of that, I was unable to browse through any email. This is unfortunate.