9.75"x13.125"x4.375" vs. 8.26"x10"x2.2"
It is 3 times the size. Probably three times the power as well.
Now, I'd gladly take the trade-off just like you - but that's you and me. The point is that, by putting "Forget mini itx cases + mobos" in your post, you're inviting a lot of flame into this thread from people who are going to be inclined to defend their purchases. They are not interchangeable products - each still has their benefits.
Also, a lot of users are all ready using a setup similar to this. The Aopen Pandora offers a similar platform at an even smaller size.
As a side note, if anybody is considering the HP product, I would suggest possibly saving money and choosing the Celeron M instead of Pentium M. Performance is only slightly degraded and power slightly increased (only important in laptop environment as this is what the chips were designed for).