Take two steps forward and one step back, that is often the situation with new ActiveSync updates. A number of Windows Mobile users have reported difficulty using the Desktop PassThru function of ActiveSync 4.2 to access the internet. Microsoft reports on the issue:
To all those who have had problems with connecting to the internet from the device when upgrading from ActiveSync 4.1 to 4.2:
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 key fix which will essentially remove this change.
The registry hack can be found in the PDAPhoneHome forum where this was posted.
(via Pocket PC Thoughts)