Despite it's many quirks, Leopard Server is still an impressive upgrade. I would say even more so than Leopard Workstation (?). One of the not-so-great changes is that you can only use the new admin tools to monitor 10.4 services (not configure) and the 10.4 admin tools installer will not work on 10.5.
In other words, you need Tiger to admin Tiger and Leopard to admin Leopard. While it seems it is indeed impossible to run the Leopard tools on Tiger, the reverse is actually quite simple. You just have to manually extract the 10.4 Server Admin Tools package contents (Archive.pax.gz).
Since I'm just learning Ruby for admin tasks, I thought it'd be neat to wrap this problem in it. So here's a script that downloads and installs the 10.4.11 admin tools package for 10.5 (under /Applications/Server 10.4).
Kind of a silly idea, but maybe there's someone out there who has a ton of Leopard clients that need to admin Tiger servers…. :-D