A few months ago I installed the Activesync 4.0 beta, and I was no longer able to sync over my wireless LAN to my desktop.
I put this down to it being a buggy new version and assumed it would be fixed... well I found out from Microsoft yesterday that this feature has been removed for security reasons!
It would have been nice to have a 'Firewall' feature defaulted to 'on', but Microsoft seem to want to plug all security holes.
This is a great pity, since it was quick and convenient to deploy and debug over the network. Apparently Bluetooth will still work though, as will of course syncronization with an Exchange Server.
http://www.pocketpcthoughts.com/forums/viewtopic.php?t=39975