With VMWare Fusion 2, VMWare has built support for installing the OS X 10.5 server operating system into the product. For various (I suppose licensing) reasons, VMWare Fusion 2 doesn’t allow one to install OS X 10.5 client – the setup wizard of Fusion would just not allow it.
The technical reason is basically that Fusion is checking for the existence of a file /System/Library/CoreServices/ServerVersion.plist on the installation media. That particular file just exists on the installation DVD for the Server version of OS X. Apparently a bunch of people have tried to create patched ISOs of the OS X installation disc, as far as I understand most of those come with various disadvantages, such as struggling to load the corrent system updates later on etc.
Another approach is to patch VMWare Fusion and just tell Fusion not to check for that file. That basically means modifying the Darwin bootloader which is located in /Library/Application Support/VMWareFusion/isoimages (darwin.iso). This file is digitally signed therefore you can’t just change or swap it. Luckily though, the certificate comes with VMWare Fusion, so that it’s possible to modify darwin.iso, sign the file and everything would be good.
A few sites have sample shell scripts for that, if you want to go the easy way – download MultiMac Helper, written by some of the technical editors of Germany’s well-know computer magazine c’t. It works really well – just run the tool once and it will patch Fusion, run it again and it will rock-back the patch. I’ve tried it – everything still works fine (for example my existing Windows VMs in Fusion). OS X 10.5 installed nicely – make sure you switch off sound and all energy saving options in your guest OS X (some people have reported issues with that).
Oh, the obvious legal disclaimer: I do not recommend that you try that at all, in particular not on any machine you have somewhat important data on. You’re on your own, don’t blame me for anything or any issue you run into and you’ve been warned. Most probably you’re even breaking some license agreements depending on the country you’re residing in. Again – you’ve been warned, don’t blame me.