Wayne it is already running Free BSD that is what Mac OS essentially is just with a nice GUI and some useful extras.
Mac OS X 10.5 server and client are Unix certified operating systems running on a modified Free BSD core as such there is no value to essentially cross grade to a less easy to use GUI and tool set.
Also Mac OS is not the only Unix with no virus, Solaris, and Free BSD have never had a virus ether nor has the cousin Linux.

"Perhaps what cripples the beast most is Apple's insistence that it run OS X, which means it is practically limited to networks of Apple client systems."
wrong. apple's problem is they are NOT a hardware company but a SOFTWARE company and is just a patch/frontend to bsd which CAN support great VT stuffs

And that's one heck of an advantage.

Of course you could just format either the Dell or the Apple system and install either Free BSD or Cent OS, and then you'd really have something.

This has been out in the current config for months and like the Mac Pro offers the best bag for buck in there respective class.

You also forget to mention what other server OS you would prefer installed as the only one different would be Windows Server since Solaris, Linux, and Free BSD would be pointless due to Mac OS also being a Unix OS with the same abilities.

The secret of enterprise IT is that no one ever pays the listed price. Consumers and those not privy to these deals don't see the true price of what is paid. Example for the same server based on the real pricing system:
2 x 2.26Ghz Intel Xeon E5520
12GB 6 x 2GB RDIMM
256MB PERC6i RAID Controller
2 x 146GB 15K RPM 2.5" SAS Drives
DVD-ROM
4 x GigE ports
Remote Management Card
Redundant Power Supplies
Rack Rails
3Yr Mission Critical 24/7 with 4 hour replacement
$4,713 + $29 shipping

Plus that price is before any haggling. Call up the Dell enterprise rep and you can easily see steeper discounts.

Kinda like airplane sales. Plane manufacturers always brag about jets sold but never reveal the final price. The analysts and talking heads always come up with a magic number based on MSRP but you can bet that airlines paid significantly less.