Since the release of the new 8-Core Mac Pro, it appears that some European online stores; most notably the UK store are offering Mac OS X 10.4 Sever free with every new Mac Pro.
The software which would usually set a consumer back £349 is listed as a free upgrade on the Mac Pro configuration page.
The offer appears to be reflected across all European stores but not the US, Canadian or Australian stores.
World of Apple contacted Apple UK sales regarding the matter and the sales person confirmed the offer.