The AMD board seems like a very situational item. The design compromises and cost increase inherent in such a design will likely make it impractical to build a new computer with AM2/DDR2 equipment with the intention of later upgrading to AM3/DDR3. The cost premium you'll pay for a motherboard like this is probably nearly as much as the price premium for just buying the AM3/DDR3 equipment now.
The best use of a board like this, in my opinion, if is you already have the AM2 processor and DDR2 memory, but need a new motherboard (yours failed or you just don't like it.) If that was true, you could get this motherboard, use your current CPU/RAM until it didn't have enough performance to make you happy, and then upgrade. You'll still be stuck with a board that wastes lots of space on tons of ram slots, but it would still probably be worth it.
The Intel board seems even less useful. Is it really a good idea to pay a premium (in money and board real estate) for the ability to upgrade to DDR3 later? As I understand it, the performance increase you get from DD3 on the Core 2 architecture just isn't all that great. Also, the cost increase on DDR3 ram isn't all that high anymore, and the motherboard will probably cost more than an equivalent motherboard without the feature. The i7 has relegated the P45 to the low to mid end of the market anyway - if you are building a new intel machine that isn't an i7, you probably won't be buying all that much ram or especially fast ram, so you could probably just spend the extra money a motherboard like this will demand and just buy the DDR3 right away. I guess it could be useful if you had a bunch of DDR2 lying around and wanted to use it for now while upgrading later, but it just doesn't seem worth it. I'd probably just keep using the DDR2 in that situation and save up for an eventual upgrade to a later platform.