Assign MAC address to DHCP Reservation

I have a situation where I have installed a St. Bernard iPrism Webfilter at our workplace. The appliance works quite nice and so far I am happy with it. One side issue that came up as part of the ongoing set up and configuration was how I would deal with outside sales members who use laptops and are in and out of the building and therefore do not sign on to the domain. I still want to filter these people and I would like to be able to identify their traffic. My question does not apply to the iPrism device or configuration, it is just provided as background knowledge for the actual question:

I believe I can accomplish what I need by assigning an IP statically to these machines. But since they are in out of my network I cannot truly static them. What I can do is set up a reservation in DHCP and assign a MAC to an IP in the reservation. By doing this they should always have the same IP when connected to our network and if they always have the same IP here I will be able to identfy their traffic and assign that traffic to them in the iPrism.

Here are my questions:
1) These laptops have two Ethernet adapters, one for the LAN connection and one for the wireless adapter. When in our building they may connect to one or the other; they could be in an office or floating about the building. Am I really going to have set aside two IP addresses for each laptop computer? Maybe its just my frugal nature and I should not care, I see DHCP is handing out two IPs for them anyway as is.
2) I have always been surious about this: when there are two adapters in a machine, one wirelless and one one cable connected LAN, and both are connected - which one is actually carrying the traffic? Is traffic split or what exactly is happening? Does one always take precedent?
3)Is what I am doing in 1 above sound or is there a better way for me to deal with this?