Okay,
POP email: Downloads messages from server onto device you are accessing the email on, and deletes them from the server (as they are now on your device). This makes accessing your POP email from multiple devices (say, a PC, a Mac, & and iPod Touch) annoying. Definitely not a Push service, must be checked when YOU want to, or set to check automatically.
IMAP: Web-based email. No matter what device you access it on, all messages STAY on the server (That's why Gmail/Hotmail/Yahoo! Mail has to offer 5GB+ of space per account), and when you access your account it just shows a list type thing of your emails. Nothing is actually downloaded from the server & placed onto your device, just downloaded from the server to your device to view the email. Definitely not a Push service, must be checked when YOU want to, or set to check automatically, like POP
Exchange/MobileMe (Push): Exactly the same as IMAP, messages are stored on the server, but instead of checking manually, when the 'cloud' (or 'server') recieves a new message, the "cloud" sort of 'pings' the device & lets it know there is new mail. This process usually takes about 5 secs to 5 or 6 mins, depending on how busy the server is.
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GMail does not support push, nor any other account type (eg Hotmail), unless it is a MS Exchange account OR a MobileMe OR Yahoo! Mail (Or similar) account, mid you the Yahoo! Mail push service on iPhone/iPod Touch is rather flaky.
But, being connected to a WiFi hotspot or 3G/EGDE service should not make any difference to how long it takes to receive mail.
Hope this helps. Sorry for the looong post !
SuperMacMan