I'm pleased to share that Activesync 4.1 has been released to the Web today. 

http://www.microsoft.com/windowsmobile/downloads/activesync41.mspx

It resolves a number of issues with Activesync 4.0 where synchronisation would not work if you used personal firewalls or network optimisation clients.

It also adds features for Windows Mobile 5.0 devices. 

You'll need Activesync 4.1 for any Windows Mobile 5.0 AKU 2 devices as well.  So for any device using the Messaging and Security Feature Pack it'll be a must!