In Lion Server Admin, under DHCP settings, it is possible to see a list of "Clients", but for me this list is nearly always empty, except in the rare occasions when I allow a machine on my network that is not statically mapped. Is it in fact the case that statically mapped IP addresses are not listed under "Clients" by default? Is there a way to see currently connected, statically mapped IP addresses?
Don't think so, no. There is a need to know which DHCP addresses are currently in use so that stale addresses can be re-issued to new clients, etc. You're looking for statically-connected users under DHCP, which is by definition not static. They will not show there.
One way to check though might be to ping the broadcast address and hope the static clients are configured to respond to pings. (IE. 192.168.1.255 on a 192.168.1.0/24 subnet).