If you're using IPSec with the Cisco client, you need to assign a pool of addresses to dole out to your remote clients. You don't do this based on MAC address. The VPN concentrator will hand out IPs from the available pool that you configure for use with IPSec clients. That said, select a range of IPs that:

a) Are not in use within your organization (i.e., don't assign IPSec NAT pools from an active DHCP range)
b) Are RFC 1918 compliant
c) Will be able to route within your network.

Reading the admin deployment guide may also be something you may want to do as well. Hint, hint.

--TH13