"Microsoft wants to alert you to an issue regarding ActiveSync 4.0 and the immediate steps the company is taking to resolve it and meet customer needs. Specifically, some users of ActiveSync� 4.0 have reported challenges when using the USB connection of their personal computer to connect to a device with Windows Mobile 5.0. In many cases, these challenges are likely caused by interoperability with desktop firewall applications or applications that manage network traffic. These applications appear to conflict with the TCP traffic between a device and the PC. An update for ActiveSync is scheduled to be available in late November, but customers may also resolve the issue manually immediately by visiting the Windows Mobile site."
There aren't that many Windows Mobile 5 devices in the wild yet, so this issue may no affect many of you (I didn't have any problems with my X51v), but there's an ActiveSync 4.0 Troubleshooting Guide on the Windows Mobile site that has some steps for you to take if you're having this issue. Nice to see Microsoft is being proactive with this issue!