Here is a brief summary of the ports and protocols used by Activesync, and their directions, to help configuring firewalls.

Direction	Source Port	Destination Port	Name	Description	Used by	Seen in
IN	any	53/udp	domain	?	?	WM2003SE
IN	68/udp	67udp	DHCP	Used to set device IP	?	WM2003SE,WM5,WM6
IN	any	990/tcp	?	handshake	odccm, vdccm	all versions
IN	5353/udp	5353/udp	mdns	Multicast DNS	avahi-daemon	WM5,WM6
IN	any	5678/tcp	rrac	Remote Replication Agent Connection	?	?
IN	any	5679/udp	dccm	Direct Cable Connect Manager	odccm,vdccm	all versions
IN	any	5721/tcp	dtpt	Desktop Passthru	?	WM5,WM6
IN	any	7438/tcp	?	?	?	WM5,WM6

Here are some firewall configuration examples for common distributions :

IPTables based

You should add the following parameters to your system configuration :

# PPP / Windows Mobile < 5
iptables -A INPUT -i ppp+ -m state --state NEW -m udp -p udp --dport 53 -j ACCEPT
iptables -A INPUT -i ppp+ -m state --state NEW -m udp -p udp --sport 68 --dport 67 -j ACCEPT
iptables -A INPUT -i ppp+ -m state --state NEW -m tcp -p tcp --dport 990 -j ACCEPT
iptables -A INPUT -i ppp+ -m state --state NEW -m tcp -p tcp --dport 5678 -j ACCEPT
iptables -A INPUT -i ppp+ -m state --state NEW -m tcp -p tcp --dport 5679 -j ACCEPT
iptables -A INPUT -i ppp+ -m state --state NEW -m tcp -p tcp --dport 7438 -j ACCEPT
# RNDIS / Windows Mobile 5+
iptables -A INPUT -i rndis+ -m state --state NEW -m udp -p udp --sport 68 --dport 67 -j ACCEPT
iptables -A INPUT -i rndis+ -m state --state NEW -m tcp -p tcp --dport 990 -j ACCEPT
iptables -A INPUT -i rndis+ -m state --state NEW -m udp -p udp --dport 5353 -j ACCEPT
iptables -A INPUT -i rndis+ -m state --state NEW -m tcp -p tcp --dport 5721 -j ACCEPT
iptables -A INPUT -i rndis+ -m state --state NEW -m udp -p udp --dport 5679 -j ACCEPT