FUD Zilla heard from an Intel engineer that all those motherboards with both DDR2 and DDR3 support aren't that great. He says it's not possible to optimize correctly for both memory types and that this will hurt overclocking performance:
This means that Combo boards will be limited in overclocking at least when it comes to memory but the same source agrees that this is the easiest way for a motherboard company to offer the easy transition.
I guess if you want to go for a motherboard that will last long time then you probably want to buy DDR2 board now and sell it in the year time when DDR3 becomes cheaper and upgrade with new chipset all over again.
If you just want the stable board and you won’t overclock, combo boards are way to go, and they will indeed offer you an easy transition.