This sounds like a great idea that I hope other manufacturers hop on. Gigabyte has a new motherboard which features both a DDR2/DDR memory controller so that it can accept DDR memory modules through the use of a converter. The performance hit is minimal according to early reports:
Gigabyte GA-PT880Pro Combo mainboard is based on VIA PT880 PRO chipset that supports Intel processors with up to 1066MHz processor system bus, PCI Express x16 as well as AGP 8x slots for graphics cards as well as sports dual-channel DDR/DDR2 memory controller that can operate with up to PC3200 (667MHz) or PC2-5400 (667MHz). Due to the nature of VIA PT880 PRO memory controller and peculiarities of Gigabyte’s mainboard design, the company supplies special converters that allow to use DDR memory in DDR2 slots, reports HKEPC web-site.