If you've upgraded to Activesync 4.2 and had problems to the Internet then a solution has just been posted on PDAPhoneHome.com...
ActiveSync version 4.2 made a change to how it interacts with the Windows networking stack to use Layered Service Providers (LSPs) on outgoing Desktop-Pass-Through connections. This allows for improved compatibility in environments where connections to the internet must be passed through these LSPs (ISA Proxy is one such example).
Unfortunately, it appears this change has caused some issues for a small segment of users who have LSPs that interfere with ActiveSync communication.
There is a registry fix which has been documented here: