As for the reasoning behind pairing a mobile cpu with a power hungry graphics card, I'm just as stumped. Perhaps there is a strong demand for a low power low noise computer that will ocasionally be used for some MAD gaming, and the two crossfire'ed cards will run idle most of the time.
So, the thing is, the Yonah is a computing beast. It runs super cool, overclocks like a dream, whups on Athlons at similar clock speeds and positively destroys anything from the current line of Pentium 4/Ds.
Just a trifle of evidence:
If you check out the Intel Top 10, you'll see me (bows) with a 2.0Ghz Yonah in a Dell Inspiron notebook (not mine) beating out P4s overclocked to 4ghz. Now, SuperPi is hardly the standard by which performance should be judged, but it's an impressive result. I also do a fair amount of H.264 encoding of DVDs on my 1.86Ghz Duo Latitude and the performance is stellar. I get >40fps on first passes and ~24fps on second passes which is right around realtime for most films. The Yonah is a beast. PLUS, the upcoming Merom (Core 2 Duo) will be pin compatible, as was mentioned, so investing in this board now will give you a pretty respectable upgrade path, as Intel is talking 20% performance improvement with zero extra heat/power for Merom. Not to mention EM64T, SSE4, and a truckload of L2 cache.
This board allows you to take advantage of next-generation performance now with mad overclock potential and still be positioned well for the future with the Merom launch coming up in a few months. With a beefy heatsink, you could probably get a good Yonah well into the 2.5-3Ghz range, which puts it in a category with top-end Athlon X2s in performance, albeit without the 64-bit extensions. You pay a premium for the board and the chip, mostly due to the niche as this is a chip generally intended for laptop OEMs, but you get plenty back, I think.
I, personally, am tickled pink that a motherboard OEM is putting out a current, well-featured mobile-chip desktop board. I can't wait for Shuttle to pack something like this into an XPC.