Check general issue #23 on Chris De Herrera's site
here.
"If you are using ActiveSync 4.2 and have a problem with Desktop Pass-through it may be caused by a Layered Service Provider (LSP) that use the Windows TCP/IP stack. This has been known to occur with ISA Server. The registry file LSPValue.reg will reverse this setting. Microsoft and the Pocket PC OEM does not support users that edit the registry."
Link to reg file
here.