Im actually having the exact same issue.
I did a fresh install of Windows 7 x64, without any updates and a minimum amount of drivers and installed iTunes. My iPhone 4 synced perfectly (tested a few times).
As soon as I installed an Antivirus, iTunes hangs soon as I connect my iPhone and un freezes soon as its removed. Updated windows fully (with SP1) and the problem doesn't change.
Ive tested with Nod32, Kaspersky, Microsoft Security Essentials and Trend Micro Titanium (just the AV versions) and all but trend caused the issue. But Trend hardly has any options at all, I wouldn't be surprised if it didn't do as much as the others.
When I had Nod32 installed, disabling it did not help, it actually had to be uninstalled to resolve the issue.
Thats too many AV programs that use to work fine with iTunes to suddenly all at the same time be causing the same issue, so it needs to be a problem on Apples end.
Like most people, setting compatibility mode (even Windows 7 compatibility mode after installing SP1) partly resolves it, but its still a bit of a crapshoot. Don't know why compatibility mode helps, since windows 7 without any updates freezes when an AV is installed, so why would Win7 compatibility mode help?
Anyone still having this issue?