Apple has published a fix/patch that may help with any some iOS4 issues you have on your iPhone 3G, 3GS, or 4 for those of you who updated software or phones. Rather than do this and by combining what I leared from Apple, I made a change on my Exchange 2007 configuration that seems to have helped based on information in this link from Microsoft. I lowered my minimum heartbeat time from 60 to 30 seconds and this seems to have helped, or I seem to be able to send attachments now. I am guessing that large attachments may fail still. Problem seems to occur on iPad iOS 3.2 also, but less frequently.
If your company assigns a .local domain via VPN then it will fail also as wireshark seems to show it trying to multicast with no failover ( or maybe just timeout ) to unicast addresses. It looks like Apple has a RFC out for this but seems like poor judgement to implement it in production.
[update]
Wondering if apple did any testing at all with enterprise users.
