We're having problems with Active Sync and iOS5 since the last few weeks. It seems that when the mobile connection is bad while checking your emails, not all mails will be synced to the iOS5 device. When you try later with a good connection the sync will not show these old mails. It seems the server already thinks it has delivered the mail to the device and will not do it again. Only if you change something (flag, read/unread) via webclient the email will show on the device.
This happened a few times in the last week (2 individual users) and its pretty irritating when you check your mails with a good connection and it shows nothing (1 specific case: 2 emails shown on iPhone for this day, in webclient 30+)
EDIT: Oops, forgot to mention: We're using Network Edition 7.2 since this weekend, but the same problem happened with the previous version.