E-mail is completely b0rked for me on 4.0 using a 3GS.
'Scorched earth' install (No backup restored - completely clean and pure) on a brand-new 3GS. (warranty replacement after my old 3GS became ridiculously unstable)
I get:
'phantom' e-mails (phone pings and vibrates, but no e-mails received)
random number displayed on the 'badge' on the mail icon (bearing no relation to the number of e-mails received)
Duplicate/triplicate e-mails
E-mails that don't get marked as 'read'. They just stay 'unread' permanently - even if left open for 1 minute.
Blank e-mails (e.g. mails that say 'no sender' 'no subject' 'this message has no content')
E-mails which have been read, randomly disappearing from the inbox.
This is all on gmail. No other e-mail accounts configured. Looks to me like I'm not the only one with this prob - seen a few posts about it on the apple forums.
Note the blank e-mails - which cannot be accessed. Tapping on them ignored. The Yougov e-mail is legible, but cannot be marked as read. The radiologists e-mail is inaccessible when tapped - I just get a message 'this message has not been downloaded from the server'. Also note the random count of '3' unread e-mails.
As an aside, this happened with 4.0 gold master on my old 3GS, but I attributed that to the fact that the phone was hosed. Seems to be a software problem. This all happens on Wi-fi as well as on cellular.
E-mail is essentially unusable for me. I'm tempted to go back to the apple store and demand that they downgrade the firmware to 3.1.3 (at least that firmware had working e-mail) or replace the phone, if they can't flash on the old firmware for me.