At .25c 1ns = Less than 3 inches. A single DDR3 DIMM is ~6 inches wide and ~1 inch tall so that's not long enough to cover a single DIMM.
Look at that picture again, including clips and those dimm slots are 7 inches wide. The top edge of a DIMM is 1 inch over the MB. And the furthest dimm is ~2 inches further from the CPU. The trace from the CPU's on board memory controller to the MB is ~1 inch. And the CPU is ~ 2 inches from the DIMMS.
EDIT: http://en.wikipedia.org/wiki/DDR3_SDRAM#mediaviewer/File:4GB... The DIMMs are not strait paths so that can easily be another inch.
Looks close to 1 foot to me which at .25c that's ~4ns. Or 40% of the minimum latency.
As to layout, that stuff is much harder than you might think as the wires can't go over each other.
That doesn't seem right.
http://www.bhphotovideo.com/bnh/controller/home?O=&sku=10887...
Looks like it might be 1ns away at .5c, which is 10% of the minimum latency and 3% of maximum (excluding refresh) latency -- and those are latencies I took off a top-of-the-line gaming RAM (my own computer is 11-11-11). It also looks like the RAM could trivially be moved closer if there were an advantage to doing so (e.g. square, stacked layout).
My point is not that the limits aren't physical, it's that the limiting physical factors are currently in silicon designs, not FR4 layouts.