I installed XenServer 5, and then added some gigabit NICs, but the server wasn't seeing them. Turned out I needed to do the following from the server console:

view plain print about
1#xe host-list
2uuid ( RO) : 9ec3de63-30fe-4575-b972-1234ed235345
3 name-label ( RW): xenserver-lfiuljac
4 name-description ( RO): Default install of XenServer
5#
6xe pif-scan host-uuid=9ec3de63-30fe-4575-b972-1234ed235345

This made the NICs show up in XenCenter.