I have two different clients, one is running Exchange 2008 and the other is running Exchange 2010. Most of the users are connected via their iPhones and the following happens.
While they are out of the office and thier workstations are off or still on but Outlook is off, they get "tons" of spam. Yet when they get to the office, log on to their workstation, bring up Outlook 2007/2010 all of that spam disapears. If the user was out of the office they would call and complain and when I went to their desk, logged on to their computer as them and opened Outlook it would start to dissapear before their eyes but I never saw of the spam via Outlook and it was not going into the Junk Folder.
It looks like as long as they leave their workstation on and Outlook open the spam doesn't reach their phones.
One client is using Vipre and the other is using Kaspersky and as long as you access your email from Outlook the spam problem is hardly noticible.